diff --git a/src/compiler/binder.ts b/src/compiler/binder.ts
index 71b5e1afbffea..4eab97f28bb27 100644
--- a/src/compiler/binder.ts
+++ b/src/compiler/binder.ts
@@ -749,6 +749,10 @@ namespace ts {
             return expr1.kind === SyntaxKind.TypeOfExpression && isNarrowableOperand((<TypeOfExpression>expr1).expression) && expr2.kind === SyntaxKind.StringLiteral;
         }
 
+        function isNarrowableInOperands(left: Expression, right: Expression) {
+            return left.kind === SyntaxKind.StringLiteral && isNarrowingExpression(right);
+        }
+
         function isNarrowingBinaryExpression(expr: BinaryExpression) {
             switch (expr.operatorToken.kind) {
                 case SyntaxKind.EqualsToken:
@@ -761,6 +765,8 @@ namespace ts {
                         isNarrowingTypeofOperands(expr.right, expr.left) || isNarrowingTypeofOperands(expr.left, expr.right);
                 case SyntaxKind.InstanceOfKeyword:
                     return isNarrowableOperand(expr.left);
+                case SyntaxKind.InKeyword:
+                    return isNarrowableInOperands(expr.left, expr.right);
                 case SyntaxKind.CommaToken:
                     return isNarrowingExpression(expr.right);
             }
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 25d967795a7ea..5404a54975dce 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -12722,6 +12722,22 @@ namespace ts {
                 return type;
             }
 
+            function isTypePresencePossible(type: Type, propName: __String, assumeTrue: boolean) {
+                const prop = getPropertyOfType(type, propName);
+                if (prop) {
+                    return (prop.flags & SymbolFlags.Optional) ? true : assumeTrue;
+                }
+                return !assumeTrue;
+            }
+
+            function narrowByInKeyword(type: Type, literal: LiteralExpression, assumeTrue: boolean) {
+                if ((type.flags & (TypeFlags.Union | TypeFlags.Object)) || (type.flags & TypeFlags.TypeParameter && (type as TypeParameter).isThisType)) {
+                    const propName = escapeLeadingUnderscores(literal.text);
+                    return filterType(type, t => isTypePresencePossible(t, propName, /* assumeTrue */ assumeTrue));
+                }
+                return type;
+            }
+
             function narrowTypeByBinaryExpression(type: Type, expr: BinaryExpression, assumeTrue: boolean): Type {
                 switch (expr.operatorToken.kind) {
                     case SyntaxKind.EqualsToken:
@@ -12757,6 +12773,12 @@ namespace ts {
                         break;
                     case SyntaxKind.InstanceOfKeyword:
                         return narrowTypeByInstanceof(type, expr, assumeTrue);
+                    case SyntaxKind.InKeyword:
+                        const target = getReferenceCandidate(expr.right);
+                        if (expr.left.kind === SyntaxKind.StringLiteral && isMatchingReference(reference, target)) {
+                            return narrowByInKeyword(type, <LiteralExpression>expr.left, assumeTrue);
+                        }
+                        break;
                     case SyntaxKind.CommaToken:
                         return narrowType(type, expr.right, assumeTrue);
                 }
diff --git a/tests/baselines/reference/fixSignatureCaching.errors.txt b/tests/baselines/reference/fixSignatureCaching.errors.txt
index 07d706a846f6e..e991908c850a3 100644
--- a/tests/baselines/reference/fixSignatureCaching.errors.txt
+++ b/tests/baselines/reference/fixSignatureCaching.errors.txt
@@ -3,6 +3,7 @@ tests/cases/conformance/fixSignatureCaching.ts(284,10): error TS2339: Property '
 tests/cases/conformance/fixSignatureCaching.ts(293,10): error TS2339: Property 'FALLBACK_PHONE' does not exist on type '{}'.
 tests/cases/conformance/fixSignatureCaching.ts(294,10): error TS2339: Property 'FALLBACK_TABLET' does not exist on type '{}'.
 tests/cases/conformance/fixSignatureCaching.ts(295,10): error TS2339: Property 'FALLBACK_MOBILE' does not exist on type '{}'.
+tests/cases/conformance/fixSignatureCaching.ts(301,17): error TS2339: Property 'isArray' does not exist on type 'never'.
 tests/cases/conformance/fixSignatureCaching.ts(330,74): error TS2339: Property 'mobileDetectRules' does not exist on type '{}'.
 tests/cases/conformance/fixSignatureCaching.ts(369,10): error TS2339: Property 'findMatch' does not exist on type '{}'.
 tests/cases/conformance/fixSignatureCaching.ts(387,10): error TS2339: Property 'findMatches' does not exist on type '{}'.
@@ -71,7 +72,7 @@ tests/cases/conformance/fixSignatureCaching.ts(982,23): error TS2304: Cannot fin
 tests/cases/conformance/fixSignatureCaching.ts(983,37): error TS2304: Cannot find name 'window'.
 
 
-==== tests/cases/conformance/fixSignatureCaching.ts (71 errors) ====
+==== tests/cases/conformance/fixSignatureCaching.ts (72 errors) ====
     // Repro from #10697
     
     (function (define, undefined) {
@@ -383,6 +384,8 @@ tests/cases/conformance/fixSignatureCaching.ts(983,37): error TS2304: Cannot fin
         isArray = 'isArray' in Array
             ? function (value) { return Object.prototype.toString.call(value) === '[object Array]'; }
             : Array.isArray;
+                    ~~~~~~~
+!!! error TS2339: Property 'isArray' does not exist on type 'never'.
     
         function equalIC(a, b) {
             return a != null && b != null && a.toLowerCase() === b.toLowerCase();
diff --git a/tests/baselines/reference/fixSignatureCaching.symbols b/tests/baselines/reference/fixSignatureCaching.symbols
index 9bae31520fd17..87cf8d0c44e14 100644
--- a/tests/baselines/reference/fixSignatureCaching.symbols
+++ b/tests/baselines/reference/fixSignatureCaching.symbols
@@ -358,9 +358,7 @@ define(function () {
 >value : Symbol(value, Decl(fixSignatureCaching.ts, 299, 20))
 
         : Array.isArray;
->Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.d.ts, --, --))
 >Array : Symbol(Array, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
->isArray : Symbol(ArrayConstructor.isArray, Decl(lib.d.ts, --, --))
 
     function equalIC(a, b) {
 >equalIC : Symbol(equalIC, Decl(fixSignatureCaching.ts, 300, 24))
diff --git a/tests/baselines/reference/fixSignatureCaching.types b/tests/baselines/reference/fixSignatureCaching.types
index 61d195e2155f8..60502cae58395 100644
--- a/tests/baselines/reference/fixSignatureCaching.types
+++ b/tests/baselines/reference/fixSignatureCaching.types
@@ -893,9 +893,9 @@ define(function () {
 >'[object Array]' : "[object Array]"
 
     isArray = 'isArray' in Array
->isArray = 'isArray' in Array        ? function (value) { return Object.prototype.toString.call(value) === '[object Array]'; }        : Array.isArray : (value: any) => boolean
+>isArray = 'isArray' in Array        ? function (value) { return Object.prototype.toString.call(value) === '[object Array]'; }        : Array.isArray : any
 >isArray : any
->'isArray' in Array        ? function (value) { return Object.prototype.toString.call(value) === '[object Array]'; }        : Array.isArray : (value: any) => boolean
+>'isArray' in Array        ? function (value) { return Object.prototype.toString.call(value) === '[object Array]'; }        : Array.isArray : any
 >'isArray' in Array : boolean
 >'isArray' : "isArray"
 >Array : ArrayConstructor
@@ -916,9 +916,9 @@ define(function () {
 >'[object Array]' : "[object Array]"
 
         : Array.isArray;
->Array.isArray : (arg: any) => arg is any[]
->Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>Array.isArray : any
+>Array : never
+>isArray : any
 
     function equalIC(a, b) {
 >equalIC : (a: any, b: any) => boolean
diff --git a/tests/baselines/reference/inKeywordTypeguard.errors.txt b/tests/baselines/reference/inKeywordTypeguard.errors.txt
new file mode 100644
index 0000000000000..914daf42a7e98
--- /dev/null
+++ b/tests/baselines/reference/inKeywordTypeguard.errors.txt
@@ -0,0 +1,159 @@
+tests/cases/compiler/inKeywordTypeguard.ts(6,11): error TS2339: Property 'b' does not exist on type 'A'.
+tests/cases/compiler/inKeywordTypeguard.ts(8,11): error TS2339: Property 'a' does not exist on type 'B'.
+tests/cases/compiler/inKeywordTypeguard.ts(14,11): error TS2339: Property 'b' does not exist on type 'A'.
+tests/cases/compiler/inKeywordTypeguard.ts(16,11): error TS2339: Property 'a' does not exist on type 'B'.
+tests/cases/compiler/inKeywordTypeguard.ts(27,11): error TS2339: Property 'b' does not exist on type 'AWithOptionalProp | BWithOptionalProp'.
+  Property 'b' does not exist on type 'AWithOptionalProp'.
+tests/cases/compiler/inKeywordTypeguard.ts(42,11): error TS2339: Property 'b' does not exist on type 'AWithMethod'.
+tests/cases/compiler/inKeywordTypeguard.ts(49,11): error TS2339: Property 'a' does not exist on type 'never'.
+tests/cases/compiler/inKeywordTypeguard.ts(50,11): error TS2339: Property 'b' does not exist on type 'never'.
+tests/cases/compiler/inKeywordTypeguard.ts(52,11): error TS2339: Property 'a' does not exist on type 'AWithMethod | BWithMethod'.
+  Property 'a' does not exist on type 'BWithMethod'.
+tests/cases/compiler/inKeywordTypeguard.ts(53,11): error TS2339: Property 'b' does not exist on type 'AWithMethod | BWithMethod'.
+  Property 'b' does not exist on type 'AWithMethod'.
+tests/cases/compiler/inKeywordTypeguard.ts(62,11): error TS2339: Property 'b' does not exist on type 'A | C | D'.
+  Property 'b' does not exist on type 'A'.
+tests/cases/compiler/inKeywordTypeguard.ts(64,11): error TS2339: Property 'a' does not exist on type 'B'.
+tests/cases/compiler/inKeywordTypeguard.ts(72,32): error TS2339: Property 'b' does not exist on type 'A'.
+tests/cases/compiler/inKeywordTypeguard.ts(74,32): error TS2339: Property 'a' does not exist on type 'B'.
+tests/cases/compiler/inKeywordTypeguard.ts(82,39): error TS2339: Property 'b' does not exist on type 'A'.
+tests/cases/compiler/inKeywordTypeguard.ts(84,39): error TS2339: Property 'a' does not exist on type 'B'.
+tests/cases/compiler/inKeywordTypeguard.ts(94,26): error TS2339: Property 'a' does not exist on type 'never'.
+
+
+==== tests/cases/compiler/inKeywordTypeguard.ts (17 errors) ====
+    class A { a: string; }
+    class B { b: string; }
+    
+    function negativeClassesTest(x: A | B) {
+        if ("a" in x) {
+            x.b = "1";
+              ~
+!!! error TS2339: Property 'b' does not exist on type 'A'.
+        } else {
+            x.a = "1";
+              ~
+!!! error TS2339: Property 'a' does not exist on type 'B'.
+        }
+    }
+    
+    function positiveClassesTest(x: A | B) {
+        if ("a" in x) {
+            x.b = "1";
+              ~
+!!! error TS2339: Property 'b' does not exist on type 'A'.
+        } else {
+            x.a = "1";
+              ~
+!!! error TS2339: Property 'a' does not exist on type 'B'.
+        }
+    }
+    
+    class AWithOptionalProp { a?: string; }
+    class BWithOptionalProp { b?: string; }
+    
+    function positiveTestClassesWithOptionalProperties(x: AWithOptionalProp | BWithOptionalProp) {
+        if ("a" in x) {
+            x.a = "1";
+        } else {
+            x.b = "1";
+              ~
+!!! error TS2339: Property 'b' does not exist on type 'AWithOptionalProp | BWithOptionalProp'.
+!!! error TS2339:   Property 'b' does not exist on type 'AWithOptionalProp'.
+        }
+    }
+    
+    class AWithMethod {
+        a(): string { return ""; }
+    }
+    
+    class BWithMethod {
+        b(): string { return ""; }
+    }
+    
+    function negativeTestClassesWithMembers(x: AWithMethod | BWithMethod) {
+        if ("a" in x) {
+            x.a();
+            x.b();
+              ~
+!!! error TS2339: Property 'b' does not exist on type 'AWithMethod'.
+        } else {
+        }
+    }
+    
+    function negativeTestClassesWithMemberMissingInBothClasses(x: AWithMethod | BWithMethod) {
+        if ("c" in x) {
+            x.a();
+              ~
+!!! error TS2339: Property 'a' does not exist on type 'never'.
+            x.b();
+              ~
+!!! error TS2339: Property 'b' does not exist on type 'never'.
+        } else {
+            x.a();
+              ~
+!!! error TS2339: Property 'a' does not exist on type 'AWithMethod | BWithMethod'.
+!!! error TS2339:   Property 'a' does not exist on type 'BWithMethod'.
+            x.b();
+              ~
+!!! error TS2339: Property 'b' does not exist on type 'AWithMethod | BWithMethod'.
+!!! error TS2339:   Property 'b' does not exist on type 'AWithMethod'.
+        }
+    }
+    
+    class C { a: string; }
+    class D { a: string; }
+    
+    function negativeMultipleClassesTest(x: A | B | C | D) {
+        if ("a" in x) {
+            x.b = "1";
+              ~
+!!! error TS2339: Property 'b' does not exist on type 'A | C | D'.
+!!! error TS2339:   Property 'b' does not exist on type 'A'.
+        } else {
+            x.a = "1";
+              ~
+!!! error TS2339: Property 'a' does not exist on type 'B'.
+        }
+    }
+    
+    class ClassWithUnionProp { prop: A | B }
+    
+    function negativePropTest(x: ClassWithUnionProp) {
+        if ("a" in x.prop) {
+            let y: string = x.prop.b;
+                                   ~
+!!! error TS2339: Property 'b' does not exist on type 'A'.
+        } else {
+            let z: string = x.prop.a;
+                                   ~
+!!! error TS2339: Property 'a' does not exist on type 'B'.
+        }
+    }
+    
+    class NegativeClassTest {
+        protected prop: A | B;
+        inThis() {
+            if ("a" in this.prop) {
+                let z: number = this.prop.b;
+                                          ~
+!!! error TS2339: Property 'b' does not exist on type 'A'.
+            } else {
+                let y: string = this.prop.a;
+                                          ~
+!!! error TS2339: Property 'a' does not exist on type 'B'.
+            }
+        }
+    }
+    
+    class UnreachableCodeDetection {
+        a: string;
+        inThis() {
+            if ("a" in this) {
+            } else {
+                let y = this.a;
+                             ~
+!!! error TS2339: Property 'a' does not exist on type 'never'.
+            }
+        }
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/inKeywordTypeguard.js b/tests/baselines/reference/inKeywordTypeguard.js
new file mode 100644
index 0000000000000..41ad27f96dff5
--- /dev/null
+++ b/tests/baselines/reference/inKeywordTypeguard.js
@@ -0,0 +1,230 @@
+//// [inKeywordTypeguard.ts]
+class A { a: string; }
+class B { b: string; }
+
+function negativeClassesTest(x: A | B) {
+    if ("a" in x) {
+        x.b = "1";
+    } else {
+        x.a = "1";
+    }
+}
+
+function positiveClassesTest(x: A | B) {
+    if ("a" in x) {
+        x.b = "1";
+    } else {
+        x.a = "1";
+    }
+}
+
+class AWithOptionalProp { a?: string; }
+class BWithOptionalProp { b?: string; }
+
+function positiveTestClassesWithOptionalProperties(x: AWithOptionalProp | BWithOptionalProp) {
+    if ("a" in x) {
+        x.a = "1";
+    } else {
+        x.b = "1";
+    }
+}
+
+class AWithMethod {
+    a(): string { return ""; }
+}
+
+class BWithMethod {
+    b(): string { return ""; }
+}
+
+function negativeTestClassesWithMembers(x: AWithMethod | BWithMethod) {
+    if ("a" in x) {
+        x.a();
+        x.b();
+    } else {
+    }
+}
+
+function negativeTestClassesWithMemberMissingInBothClasses(x: AWithMethod | BWithMethod) {
+    if ("c" in x) {
+        x.a();
+        x.b();
+    } else {
+        x.a();
+        x.b();
+    }
+}
+
+class C { a: string; }
+class D { a: string; }
+
+function negativeMultipleClassesTest(x: A | B | C | D) {
+    if ("a" in x) {
+        x.b = "1";
+    } else {
+        x.a = "1";
+    }
+}
+
+class ClassWithUnionProp { prop: A | B }
+
+function negativePropTest(x: ClassWithUnionProp) {
+    if ("a" in x.prop) {
+        let y: string = x.prop.b;
+    } else {
+        let z: string = x.prop.a;
+    }
+}
+
+class NegativeClassTest {
+    protected prop: A | B;
+    inThis() {
+        if ("a" in this.prop) {
+            let z: number = this.prop.b;
+        } else {
+            let y: string = this.prop.a;
+        }
+    }
+}
+
+class UnreachableCodeDetection {
+    a: string;
+    inThis() {
+        if ("a" in this) {
+        } else {
+            let y = this.a;
+        }
+    }
+}
+
+//// [inKeywordTypeguard.js]
+var A = /** @class */ (function () {
+    function A() {
+    }
+    return A;
+}());
+var B = /** @class */ (function () {
+    function B() {
+    }
+    return B;
+}());
+function negativeClassesTest(x) {
+    if ("a" in x) {
+        x.b = "1";
+    }
+    else {
+        x.a = "1";
+    }
+}
+function positiveClassesTest(x) {
+    if ("a" in x) {
+        x.b = "1";
+    }
+    else {
+        x.a = "1";
+    }
+}
+var AWithOptionalProp = /** @class */ (function () {
+    function AWithOptionalProp() {
+    }
+    return AWithOptionalProp;
+}());
+var BWithOptionalProp = /** @class */ (function () {
+    function BWithOptionalProp() {
+    }
+    return BWithOptionalProp;
+}());
+function positiveTestClassesWithOptionalProperties(x) {
+    if ("a" in x) {
+        x.a = "1";
+    }
+    else {
+        x.b = "1";
+    }
+}
+var AWithMethod = /** @class */ (function () {
+    function AWithMethod() {
+    }
+    AWithMethod.prototype.a = function () { return ""; };
+    return AWithMethod;
+}());
+var BWithMethod = /** @class */ (function () {
+    function BWithMethod() {
+    }
+    BWithMethod.prototype.b = function () { return ""; };
+    return BWithMethod;
+}());
+function negativeTestClassesWithMembers(x) {
+    if ("a" in x) {
+        x.a();
+        x.b();
+    }
+    else {
+    }
+}
+function negativeTestClassesWithMemberMissingInBothClasses(x) {
+    if ("c" in x) {
+        x.a();
+        x.b();
+    }
+    else {
+        x.a();
+        x.b();
+    }
+}
+var C = /** @class */ (function () {
+    function C() {
+    }
+    return C;
+}());
+var D = /** @class */ (function () {
+    function D() {
+    }
+    return D;
+}());
+function negativeMultipleClassesTest(x) {
+    if ("a" in x) {
+        x.b = "1";
+    }
+    else {
+        x.a = "1";
+    }
+}
+var ClassWithUnionProp = /** @class */ (function () {
+    function ClassWithUnionProp() {
+    }
+    return ClassWithUnionProp;
+}());
+function negativePropTest(x) {
+    if ("a" in x.prop) {
+        var y = x.prop.b;
+    }
+    else {
+        var z = x.prop.a;
+    }
+}
+var NegativeClassTest = /** @class */ (function () {
+    function NegativeClassTest() {
+    }
+    NegativeClassTest.prototype.inThis = function () {
+        if ("a" in this.prop) {
+            var z = this.prop.b;
+        }
+        else {
+            var y = this.prop.a;
+        }
+    };
+    return NegativeClassTest;
+}());
+var UnreachableCodeDetection = /** @class */ (function () {
+    function UnreachableCodeDetection() {
+    }
+    UnreachableCodeDetection.prototype.inThis = function () {
+        if ("a" in this) {
+        }
+        else {
+            var y = this.a;
+        }
+    };
+    return UnreachableCodeDetection;
+}());
diff --git a/tests/baselines/reference/inKeywordTypeguard.symbols b/tests/baselines/reference/inKeywordTypeguard.symbols
new file mode 100644
index 0000000000000..c73f6f4901de5
--- /dev/null
+++ b/tests/baselines/reference/inKeywordTypeguard.symbols
@@ -0,0 +1,241 @@
+=== tests/cases/compiler/inKeywordTypeguard.ts ===
+class A { a: string; }
+>A : Symbol(A, Decl(inKeywordTypeguard.ts, 0, 0))
+>a : Symbol(A.a, Decl(inKeywordTypeguard.ts, 0, 9))
+
+class B { b: string; }
+>B : Symbol(B, Decl(inKeywordTypeguard.ts, 0, 22))
+>b : Symbol(B.b, Decl(inKeywordTypeguard.ts, 1, 9))
+
+function negativeClassesTest(x: A | B) {
+>negativeClassesTest : Symbol(negativeClassesTest, Decl(inKeywordTypeguard.ts, 1, 22))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 3, 29))
+>A : Symbol(A, Decl(inKeywordTypeguard.ts, 0, 0))
+>B : Symbol(B, Decl(inKeywordTypeguard.ts, 0, 22))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 3, 29))
+
+        x.b = "1";
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 3, 29))
+
+    } else {
+        x.a = "1";
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 3, 29))
+    }
+}
+
+function positiveClassesTest(x: A | B) {
+>positiveClassesTest : Symbol(positiveClassesTest, Decl(inKeywordTypeguard.ts, 9, 1))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 11, 29))
+>A : Symbol(A, Decl(inKeywordTypeguard.ts, 0, 0))
+>B : Symbol(B, Decl(inKeywordTypeguard.ts, 0, 22))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 11, 29))
+
+        x.b = "1";
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 11, 29))
+
+    } else {
+        x.a = "1";
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 11, 29))
+    }
+}
+
+class AWithOptionalProp { a?: string; }
+>AWithOptionalProp : Symbol(AWithOptionalProp, Decl(inKeywordTypeguard.ts, 17, 1))
+>a : Symbol(AWithOptionalProp.a, Decl(inKeywordTypeguard.ts, 19, 25))
+
+class BWithOptionalProp { b?: string; }
+>BWithOptionalProp : Symbol(BWithOptionalProp, Decl(inKeywordTypeguard.ts, 19, 39))
+>b : Symbol(BWithOptionalProp.b, Decl(inKeywordTypeguard.ts, 20, 25))
+
+function positiveTestClassesWithOptionalProperties(x: AWithOptionalProp | BWithOptionalProp) {
+>positiveTestClassesWithOptionalProperties : Symbol(positiveTestClassesWithOptionalProperties, Decl(inKeywordTypeguard.ts, 20, 39))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 22, 51))
+>AWithOptionalProp : Symbol(AWithOptionalProp, Decl(inKeywordTypeguard.ts, 17, 1))
+>BWithOptionalProp : Symbol(BWithOptionalProp, Decl(inKeywordTypeguard.ts, 19, 39))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 22, 51))
+
+        x.a = "1";
+>x.a : Symbol(AWithOptionalProp.a, Decl(inKeywordTypeguard.ts, 19, 25))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 22, 51))
+>a : Symbol(AWithOptionalProp.a, Decl(inKeywordTypeguard.ts, 19, 25))
+
+    } else {
+        x.b = "1";
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 22, 51))
+    }
+}
+
+class AWithMethod {
+>AWithMethod : Symbol(AWithMethod, Decl(inKeywordTypeguard.ts, 28, 1))
+
+    a(): string { return ""; }
+>a : Symbol(AWithMethod.a, Decl(inKeywordTypeguard.ts, 30, 19))
+}
+
+class BWithMethod {
+>BWithMethod : Symbol(BWithMethod, Decl(inKeywordTypeguard.ts, 32, 1))
+
+    b(): string { return ""; }
+>b : Symbol(BWithMethod.b, Decl(inKeywordTypeguard.ts, 34, 19))
+}
+
+function negativeTestClassesWithMembers(x: AWithMethod | BWithMethod) {
+>negativeTestClassesWithMembers : Symbol(negativeTestClassesWithMembers, Decl(inKeywordTypeguard.ts, 36, 1))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 38, 40))
+>AWithMethod : Symbol(AWithMethod, Decl(inKeywordTypeguard.ts, 28, 1))
+>BWithMethod : Symbol(BWithMethod, Decl(inKeywordTypeguard.ts, 32, 1))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 38, 40))
+
+        x.a();
+>x.a : Symbol(AWithMethod.a, Decl(inKeywordTypeguard.ts, 30, 19))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 38, 40))
+>a : Symbol(AWithMethod.a, Decl(inKeywordTypeguard.ts, 30, 19))
+
+        x.b();
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 38, 40))
+
+    } else {
+    }
+}
+
+function negativeTestClassesWithMemberMissingInBothClasses(x: AWithMethod | BWithMethod) {
+>negativeTestClassesWithMemberMissingInBothClasses : Symbol(negativeTestClassesWithMemberMissingInBothClasses, Decl(inKeywordTypeguard.ts, 44, 1))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 46, 59))
+>AWithMethod : Symbol(AWithMethod, Decl(inKeywordTypeguard.ts, 28, 1))
+>BWithMethod : Symbol(BWithMethod, Decl(inKeywordTypeguard.ts, 32, 1))
+
+    if ("c" in x) {
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 46, 59))
+
+        x.a();
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 46, 59))
+
+        x.b();
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 46, 59))
+
+    } else {
+        x.a();
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 46, 59))
+
+        x.b();
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 46, 59))
+    }
+}
+
+class C { a: string; }
+>C : Symbol(C, Decl(inKeywordTypeguard.ts, 54, 1))
+>a : Symbol(C.a, Decl(inKeywordTypeguard.ts, 56, 9))
+
+class D { a: string; }
+>D : Symbol(D, Decl(inKeywordTypeguard.ts, 56, 22))
+>a : Symbol(D.a, Decl(inKeywordTypeguard.ts, 57, 9))
+
+function negativeMultipleClassesTest(x: A | B | C | D) {
+>negativeMultipleClassesTest : Symbol(negativeMultipleClassesTest, Decl(inKeywordTypeguard.ts, 57, 22))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 59, 37))
+>A : Symbol(A, Decl(inKeywordTypeguard.ts, 0, 0))
+>B : Symbol(B, Decl(inKeywordTypeguard.ts, 0, 22))
+>C : Symbol(C, Decl(inKeywordTypeguard.ts, 54, 1))
+>D : Symbol(D, Decl(inKeywordTypeguard.ts, 56, 22))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 59, 37))
+
+        x.b = "1";
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 59, 37))
+
+    } else {
+        x.a = "1";
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 59, 37))
+    }
+}
+
+class ClassWithUnionProp { prop: A | B }
+>ClassWithUnionProp : Symbol(ClassWithUnionProp, Decl(inKeywordTypeguard.ts, 65, 1))
+>prop : Symbol(ClassWithUnionProp.prop, Decl(inKeywordTypeguard.ts, 67, 26))
+>A : Symbol(A, Decl(inKeywordTypeguard.ts, 0, 0))
+>B : Symbol(B, Decl(inKeywordTypeguard.ts, 0, 22))
+
+function negativePropTest(x: ClassWithUnionProp) {
+>negativePropTest : Symbol(negativePropTest, Decl(inKeywordTypeguard.ts, 67, 40))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 69, 26))
+>ClassWithUnionProp : Symbol(ClassWithUnionProp, Decl(inKeywordTypeguard.ts, 65, 1))
+
+    if ("a" in x.prop) {
+>x.prop : Symbol(ClassWithUnionProp.prop, Decl(inKeywordTypeguard.ts, 67, 26))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 69, 26))
+>prop : Symbol(ClassWithUnionProp.prop, Decl(inKeywordTypeguard.ts, 67, 26))
+
+        let y: string = x.prop.b;
+>y : Symbol(y, Decl(inKeywordTypeguard.ts, 71, 11))
+>x.prop : Symbol(ClassWithUnionProp.prop, Decl(inKeywordTypeguard.ts, 67, 26))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 69, 26))
+>prop : Symbol(ClassWithUnionProp.prop, Decl(inKeywordTypeguard.ts, 67, 26))
+
+    } else {
+        let z: string = x.prop.a;
+>z : Symbol(z, Decl(inKeywordTypeguard.ts, 73, 11))
+>x.prop : Symbol(ClassWithUnionProp.prop, Decl(inKeywordTypeguard.ts, 67, 26))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 69, 26))
+>prop : Symbol(ClassWithUnionProp.prop, Decl(inKeywordTypeguard.ts, 67, 26))
+    }
+}
+
+class NegativeClassTest {
+>NegativeClassTest : Symbol(NegativeClassTest, Decl(inKeywordTypeguard.ts, 75, 1))
+
+    protected prop: A | B;
+>prop : Symbol(NegativeClassTest.prop, Decl(inKeywordTypeguard.ts, 77, 25))
+>A : Symbol(A, Decl(inKeywordTypeguard.ts, 0, 0))
+>B : Symbol(B, Decl(inKeywordTypeguard.ts, 0, 22))
+
+    inThis() {
+>inThis : Symbol(NegativeClassTest.inThis, Decl(inKeywordTypeguard.ts, 78, 26))
+
+        if ("a" in this.prop) {
+>this.prop : Symbol(NegativeClassTest.prop, Decl(inKeywordTypeguard.ts, 77, 25))
+>this : Symbol(NegativeClassTest, Decl(inKeywordTypeguard.ts, 75, 1))
+>prop : Symbol(NegativeClassTest.prop, Decl(inKeywordTypeguard.ts, 77, 25))
+
+            let z: number = this.prop.b;
+>z : Symbol(z, Decl(inKeywordTypeguard.ts, 81, 15))
+>this.prop : Symbol(NegativeClassTest.prop, Decl(inKeywordTypeguard.ts, 77, 25))
+>this : Symbol(NegativeClassTest, Decl(inKeywordTypeguard.ts, 75, 1))
+>prop : Symbol(NegativeClassTest.prop, Decl(inKeywordTypeguard.ts, 77, 25))
+
+        } else {
+            let y: string = this.prop.a;
+>y : Symbol(y, Decl(inKeywordTypeguard.ts, 83, 15))
+>this.prop : Symbol(NegativeClassTest.prop, Decl(inKeywordTypeguard.ts, 77, 25))
+>this : Symbol(NegativeClassTest, Decl(inKeywordTypeguard.ts, 75, 1))
+>prop : Symbol(NegativeClassTest.prop, Decl(inKeywordTypeguard.ts, 77, 25))
+        }
+    }
+}
+
+class UnreachableCodeDetection {
+>UnreachableCodeDetection : Symbol(UnreachableCodeDetection, Decl(inKeywordTypeguard.ts, 86, 1))
+
+    a: string;
+>a : Symbol(UnreachableCodeDetection.a, Decl(inKeywordTypeguard.ts, 88, 32))
+
+    inThis() {
+>inThis : Symbol(UnreachableCodeDetection.inThis, Decl(inKeywordTypeguard.ts, 89, 14))
+
+        if ("a" in this) {
+>this : Symbol(UnreachableCodeDetection, Decl(inKeywordTypeguard.ts, 86, 1))
+
+        } else {
+            let y = this.a;
+>y : Symbol(y, Decl(inKeywordTypeguard.ts, 93, 15))
+        }
+    }
+}
diff --git a/tests/baselines/reference/inKeywordTypeguard.types b/tests/baselines/reference/inKeywordTypeguard.types
new file mode 100644
index 0000000000000..3f9e8fd063869
--- /dev/null
+++ b/tests/baselines/reference/inKeywordTypeguard.types
@@ -0,0 +1,318 @@
+=== tests/cases/compiler/inKeywordTypeguard.ts ===
+class A { a: string; }
+>A : A
+>a : string
+
+class B { b: string; }
+>B : B
+>b : string
+
+function negativeClassesTest(x: A | B) {
+>negativeClassesTest : (x: A | B) => void
+>x : A | B
+>A : A
+>B : B
+
+    if ("a" in x) {
+>"a" in x : boolean
+>"a" : "a"
+>x : A | B
+
+        x.b = "1";
+>x.b = "1" : "1"
+>x.b : any
+>x : A
+>b : any
+>"1" : "1"
+
+    } else {
+        x.a = "1";
+>x.a = "1" : "1"
+>x.a : any
+>x : B
+>a : any
+>"1" : "1"
+    }
+}
+
+function positiveClassesTest(x: A | B) {
+>positiveClassesTest : (x: A | B) => void
+>x : A | B
+>A : A
+>B : B
+
+    if ("a" in x) {
+>"a" in x : boolean
+>"a" : "a"
+>x : A | B
+
+        x.b = "1";
+>x.b = "1" : "1"
+>x.b : any
+>x : A
+>b : any
+>"1" : "1"
+
+    } else {
+        x.a = "1";
+>x.a = "1" : "1"
+>x.a : any
+>x : B
+>a : any
+>"1" : "1"
+    }
+}
+
+class AWithOptionalProp { a?: string; }
+>AWithOptionalProp : AWithOptionalProp
+>a : string
+
+class BWithOptionalProp { b?: string; }
+>BWithOptionalProp : BWithOptionalProp
+>b : string
+
+function positiveTestClassesWithOptionalProperties(x: AWithOptionalProp | BWithOptionalProp) {
+>positiveTestClassesWithOptionalProperties : (x: AWithOptionalProp | BWithOptionalProp) => void
+>x : AWithOptionalProp | BWithOptionalProp
+>AWithOptionalProp : AWithOptionalProp
+>BWithOptionalProp : BWithOptionalProp
+
+    if ("a" in x) {
+>"a" in x : boolean
+>"a" : "a"
+>x : AWithOptionalProp | BWithOptionalProp
+
+        x.a = "1";
+>x.a = "1" : "1"
+>x.a : string
+>x : AWithOptionalProp
+>a : string
+>"1" : "1"
+
+    } else {
+        x.b = "1";
+>x.b = "1" : "1"
+>x.b : any
+>x : AWithOptionalProp | BWithOptionalProp
+>b : any
+>"1" : "1"
+    }
+}
+
+class AWithMethod {
+>AWithMethod : AWithMethod
+
+    a(): string { return ""; }
+>a : () => string
+>"" : ""
+}
+
+class BWithMethod {
+>BWithMethod : BWithMethod
+
+    b(): string { return ""; }
+>b : () => string
+>"" : ""
+}
+
+function negativeTestClassesWithMembers(x: AWithMethod | BWithMethod) {
+>negativeTestClassesWithMembers : (x: AWithMethod | BWithMethod) => void
+>x : AWithMethod | BWithMethod
+>AWithMethod : AWithMethod
+>BWithMethod : BWithMethod
+
+    if ("a" in x) {
+>"a" in x : boolean
+>"a" : "a"
+>x : AWithMethod | BWithMethod
+
+        x.a();
+>x.a() : string
+>x.a : () => string
+>x : AWithMethod
+>a : () => string
+
+        x.b();
+>x.b() : any
+>x.b : any
+>x : AWithMethod
+>b : any
+
+    } else {
+    }
+}
+
+function negativeTestClassesWithMemberMissingInBothClasses(x: AWithMethod | BWithMethod) {
+>negativeTestClassesWithMemberMissingInBothClasses : (x: AWithMethod | BWithMethod) => void
+>x : AWithMethod | BWithMethod
+>AWithMethod : AWithMethod
+>BWithMethod : BWithMethod
+
+    if ("c" in x) {
+>"c" in x : boolean
+>"c" : "c"
+>x : AWithMethod | BWithMethod
+
+        x.a();
+>x.a() : any
+>x.a : any
+>x : never
+>a : any
+
+        x.b();
+>x.b() : any
+>x.b : any
+>x : never
+>b : any
+
+    } else {
+        x.a();
+>x.a() : any
+>x.a : any
+>x : AWithMethod | BWithMethod
+>a : any
+
+        x.b();
+>x.b() : any
+>x.b : any
+>x : AWithMethod | BWithMethod
+>b : any
+    }
+}
+
+class C { a: string; }
+>C : C
+>a : string
+
+class D { a: string; }
+>D : D
+>a : string
+
+function negativeMultipleClassesTest(x: A | B | C | D) {
+>negativeMultipleClassesTest : (x: A | B | C | D) => void
+>x : A | B | C | D
+>A : A
+>B : B
+>C : C
+>D : D
+
+    if ("a" in x) {
+>"a" in x : boolean
+>"a" : "a"
+>x : A | B | C | D
+
+        x.b = "1";
+>x.b = "1" : "1"
+>x.b : any
+>x : A | C | D
+>b : any
+>"1" : "1"
+
+    } else {
+        x.a = "1";
+>x.a = "1" : "1"
+>x.a : any
+>x : B
+>a : any
+>"1" : "1"
+    }
+}
+
+class ClassWithUnionProp { prop: A | B }
+>ClassWithUnionProp : ClassWithUnionProp
+>prop : A | B
+>A : A
+>B : B
+
+function negativePropTest(x: ClassWithUnionProp) {
+>negativePropTest : (x: ClassWithUnionProp) => void
+>x : ClassWithUnionProp
+>ClassWithUnionProp : ClassWithUnionProp
+
+    if ("a" in x.prop) {
+>"a" in x.prop : boolean
+>"a" : "a"
+>x.prop : A | B
+>x : ClassWithUnionProp
+>prop : A | B
+
+        let y: string = x.prop.b;
+>y : string
+>x.prop.b : any
+>x.prop : A
+>x : ClassWithUnionProp
+>prop : A
+>b : any
+
+    } else {
+        let z: string = x.prop.a;
+>z : string
+>x.prop.a : any
+>x.prop : B
+>x : ClassWithUnionProp
+>prop : B
+>a : any
+    }
+}
+
+class NegativeClassTest {
+>NegativeClassTest : NegativeClassTest
+
+    protected prop: A | B;
+>prop : A | B
+>A : A
+>B : B
+
+    inThis() {
+>inThis : () => void
+
+        if ("a" in this.prop) {
+>"a" in this.prop : boolean
+>"a" : "a"
+>this.prop : A | B
+>this : this
+>prop : A | B
+
+            let z: number = this.prop.b;
+>z : number
+>this.prop.b : any
+>this.prop : A
+>this : this
+>prop : A
+>b : any
+
+        } else {
+            let y: string = this.prop.a;
+>y : string
+>this.prop.a : any
+>this.prop : B
+>this : this
+>prop : B
+>a : any
+        }
+    }
+}
+
+class UnreachableCodeDetection {
+>UnreachableCodeDetection : UnreachableCodeDetection
+
+    a: string;
+>a : string
+
+    inThis() {
+>inThis : () => void
+
+        if ("a" in this) {
+>"a" in this : boolean
+>"a" : "a"
+>this : this
+
+        } else {
+            let y = this.a;
+>y : any
+>this.a : any
+>this : never
+>a : any
+        }
+    }
+}
diff --git a/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.js b/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.js
new file mode 100644
index 0000000000000..a2579328646e6
--- /dev/null
+++ b/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.js
@@ -0,0 +1,218 @@
+//// [typeGuardOfFromPropNameInUnionType.ts]
+class A { a: string; }
+class B { b: number; }
+class C { b: Object; }
+class D { a: Date; }
+
+function namedClasses(x: A | B) {
+    if ("a" in x) {
+        x.a = "1";
+    } else {
+        x.b = 1;
+    }
+}
+
+function multipleClasses(x: A | B | C | D) {
+    if ("a" in x) {
+        let y: string | Date = x.a;
+    } else {
+        let z: number | Object = x.b;
+    }
+}
+
+function anonymousClasses(x: { a: string; } | { b: number; }) {
+    if ("a" in x) {
+        let y: string = x.a;
+    } else {
+        let z: number = x.b;
+    }
+}
+
+class AWithOptionalProp { a?: string; }
+class BWithOptionalProp { b?: string; }
+
+function positiveTestClassesWithOptionalProperties(x: AWithOptionalProp | BWithOptionalProp) {
+    if ("a" in x) {
+        x.a = "1";
+    } else {
+        const y: string = x instanceof AWithOptionalProp
+            ? x.a
+            : x.b
+    }
+}
+
+function inParenthesizedExpression(x: A | B) {
+    if ("a" in (x)) {
+        let y: string = x.a;
+    } else {
+        let z: number = x.b;
+    }
+}
+
+class ClassWithUnionProp { prop: A | B; }
+
+function inProperty(x: ClassWithUnionProp) {
+    if ("a" in x.prop) {
+        let y: string = x.prop.a;
+    } else {
+        let z: number = x.prop.b;
+    }
+}
+
+class NestedClassWithProp { outer: ClassWithUnionProp; }
+
+function innestedProperty(x: NestedClassWithProp) {
+    if ("a" in x.outer.prop) {
+        let y: string = x.outer.prop.a;
+    } else {
+        let z: number = x.outer.prop.b;
+    }
+}
+
+class InMemberOfClass {
+    protected prop: A | B;
+    inThis() {
+        if ("a" in this.prop) {
+            let y: string = this.prop.a;
+        } else {
+            let z: number = this.prop.b;
+        }
+    }
+}
+
+// added for completeness
+class SelfAssert {
+    a: string;
+    inThis() {
+        if ("a" in this) {
+            let y: string = this.a;
+        } else {
+        }
+    }
+}
+
+//// [typeGuardOfFromPropNameInUnionType.js]
+var A = /** @class */ (function () {
+    function A() {
+    }
+    return A;
+}());
+var B = /** @class */ (function () {
+    function B() {
+    }
+    return B;
+}());
+var C = /** @class */ (function () {
+    function C() {
+    }
+    return C;
+}());
+var D = /** @class */ (function () {
+    function D() {
+    }
+    return D;
+}());
+function namedClasses(x) {
+    if ("a" in x) {
+        x.a = "1";
+    }
+    else {
+        x.b = 1;
+    }
+}
+function multipleClasses(x) {
+    if ("a" in x) {
+        var y = x.a;
+    }
+    else {
+        var z = x.b;
+    }
+}
+function anonymousClasses(x) {
+    if ("a" in x) {
+        var y = x.a;
+    }
+    else {
+        var z = x.b;
+    }
+}
+var AWithOptionalProp = /** @class */ (function () {
+    function AWithOptionalProp() {
+    }
+    return AWithOptionalProp;
+}());
+var BWithOptionalProp = /** @class */ (function () {
+    function BWithOptionalProp() {
+    }
+    return BWithOptionalProp;
+}());
+function positiveTestClassesWithOptionalProperties(x) {
+    if ("a" in x) {
+        x.a = "1";
+    }
+    else {
+        var y = x instanceof AWithOptionalProp
+            ? x.a
+            : x.b;
+    }
+}
+function inParenthesizedExpression(x) {
+    if ("a" in (x)) {
+        var y = x.a;
+    }
+    else {
+        var z = x.b;
+    }
+}
+var ClassWithUnionProp = /** @class */ (function () {
+    function ClassWithUnionProp() {
+    }
+    return ClassWithUnionProp;
+}());
+function inProperty(x) {
+    if ("a" in x.prop) {
+        var y = x.prop.a;
+    }
+    else {
+        var z = x.prop.b;
+    }
+}
+var NestedClassWithProp = /** @class */ (function () {
+    function NestedClassWithProp() {
+    }
+    return NestedClassWithProp;
+}());
+function innestedProperty(x) {
+    if ("a" in x.outer.prop) {
+        var y = x.outer.prop.a;
+    }
+    else {
+        var z = x.outer.prop.b;
+    }
+}
+var InMemberOfClass = /** @class */ (function () {
+    function InMemberOfClass() {
+    }
+    InMemberOfClass.prototype.inThis = function () {
+        if ("a" in this.prop) {
+            var y = this.prop.a;
+        }
+        else {
+            var z = this.prop.b;
+        }
+    };
+    return InMemberOfClass;
+}());
+// added for completeness
+var SelfAssert = /** @class */ (function () {
+    function SelfAssert() {
+    }
+    SelfAssert.prototype.inThis = function () {
+        if ("a" in this) {
+            var y = this.a;
+        }
+        else {
+        }
+    };
+    return SelfAssert;
+}());
diff --git a/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.symbols b/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.symbols
new file mode 100644
index 0000000000000..72fb3df2fc571
--- /dev/null
+++ b/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.symbols
@@ -0,0 +1,291 @@
+=== tests/cases/conformance/expressions/typeGuards/typeGuardOfFromPropNameInUnionType.ts ===
+class A { a: string; }
+>A : Symbol(A, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 0))
+>a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+
+class B { b: number; }
+>B : Symbol(B, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 22))
+>b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+
+class C { b: Object; }
+>C : Symbol(C, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 22))
+>b : Symbol(C.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 2, 9))
+>Object : Symbol(Object, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+
+class D { a: Date; }
+>D : Symbol(D, Decl(typeGuardOfFromPropNameInUnionType.ts, 2, 22))
+>a : Symbol(D.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 3, 9))
+>Date : Symbol(Date, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+
+function namedClasses(x: A | B) {
+>namedClasses : Symbol(namedClasses, Decl(typeGuardOfFromPropNameInUnionType.ts, 3, 20))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 5, 22))
+>A : Symbol(A, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 0))
+>B : Symbol(B, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 22))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 5, 22))
+
+        x.a = "1";
+>x.a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 5, 22))
+>a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+
+    } else {
+        x.b = 1;
+>x.b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 5, 22))
+>b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+    }
+}
+
+function multipleClasses(x: A | B | C | D) {
+>multipleClasses : Symbol(multipleClasses, Decl(typeGuardOfFromPropNameInUnionType.ts, 11, 1))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 13, 25))
+>A : Symbol(A, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 0))
+>B : Symbol(B, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 22))
+>C : Symbol(C, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 22))
+>D : Symbol(D, Decl(typeGuardOfFromPropNameInUnionType.ts, 2, 22))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 13, 25))
+
+        let y: string | Date = x.a;
+>y : Symbol(y, Decl(typeGuardOfFromPropNameInUnionType.ts, 15, 11))
+>Date : Symbol(Date, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+>x.a : Symbol(a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9), Decl(typeGuardOfFromPropNameInUnionType.ts, 3, 9))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 13, 25))
+>a : Symbol(a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9), Decl(typeGuardOfFromPropNameInUnionType.ts, 3, 9))
+
+    } else {
+        let z: number | Object = x.b;
+>z : Symbol(z, Decl(typeGuardOfFromPropNameInUnionType.ts, 17, 11))
+>Object : Symbol(Object, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+>x.b : Symbol(b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9), Decl(typeGuardOfFromPropNameInUnionType.ts, 2, 9))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 13, 25))
+>b : Symbol(b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9), Decl(typeGuardOfFromPropNameInUnionType.ts, 2, 9))
+    }
+}
+
+function anonymousClasses(x: { a: string; } | { b: number; }) {
+>anonymousClasses : Symbol(anonymousClasses, Decl(typeGuardOfFromPropNameInUnionType.ts, 19, 1))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 21, 26))
+>a : Symbol(a, Decl(typeGuardOfFromPropNameInUnionType.ts, 21, 30))
+>b : Symbol(b, Decl(typeGuardOfFromPropNameInUnionType.ts, 21, 47))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 21, 26))
+
+        let y: string = x.a;
+>y : Symbol(y, Decl(typeGuardOfFromPropNameInUnionType.ts, 23, 11))
+>x.a : Symbol(a, Decl(typeGuardOfFromPropNameInUnionType.ts, 21, 30))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 21, 26))
+>a : Symbol(a, Decl(typeGuardOfFromPropNameInUnionType.ts, 21, 30))
+
+    } else {
+        let z: number = x.b;
+>z : Symbol(z, Decl(typeGuardOfFromPropNameInUnionType.ts, 25, 11))
+>x.b : Symbol(b, Decl(typeGuardOfFromPropNameInUnionType.ts, 21, 47))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 21, 26))
+>b : Symbol(b, Decl(typeGuardOfFromPropNameInUnionType.ts, 21, 47))
+    }
+}
+
+class AWithOptionalProp { a?: string; }
+>AWithOptionalProp : Symbol(AWithOptionalProp, Decl(typeGuardOfFromPropNameInUnionType.ts, 27, 1))
+>a : Symbol(AWithOptionalProp.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 29, 25))
+
+class BWithOptionalProp { b?: string; }
+>BWithOptionalProp : Symbol(BWithOptionalProp, Decl(typeGuardOfFromPropNameInUnionType.ts, 29, 39))
+>b : Symbol(BWithOptionalProp.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 30, 25))
+
+function positiveTestClassesWithOptionalProperties(x: AWithOptionalProp | BWithOptionalProp) {
+>positiveTestClassesWithOptionalProperties : Symbol(positiveTestClassesWithOptionalProperties, Decl(typeGuardOfFromPropNameInUnionType.ts, 30, 39))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 32, 51))
+>AWithOptionalProp : Symbol(AWithOptionalProp, Decl(typeGuardOfFromPropNameInUnionType.ts, 27, 1))
+>BWithOptionalProp : Symbol(BWithOptionalProp, Decl(typeGuardOfFromPropNameInUnionType.ts, 29, 39))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 32, 51))
+
+        x.a = "1";
+>x.a : Symbol(AWithOptionalProp.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 29, 25))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 32, 51))
+>a : Symbol(AWithOptionalProp.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 29, 25))
+
+    } else {
+        const y: string = x instanceof AWithOptionalProp
+>y : Symbol(y, Decl(typeGuardOfFromPropNameInUnionType.ts, 36, 13))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 32, 51))
+>AWithOptionalProp : Symbol(AWithOptionalProp, Decl(typeGuardOfFromPropNameInUnionType.ts, 27, 1))
+
+            ? x.a
+>x.a : Symbol(AWithOptionalProp.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 29, 25))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 32, 51))
+>a : Symbol(AWithOptionalProp.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 29, 25))
+
+            : x.b
+>x.b : Symbol(BWithOptionalProp.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 30, 25))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 32, 51))
+>b : Symbol(BWithOptionalProp.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 30, 25))
+    }
+}
+
+function inParenthesizedExpression(x: A | B) {
+>inParenthesizedExpression : Symbol(inParenthesizedExpression, Decl(typeGuardOfFromPropNameInUnionType.ts, 40, 1))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 42, 35))
+>A : Symbol(A, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 0))
+>B : Symbol(B, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 22))
+
+    if ("a" in (x)) {
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 42, 35))
+
+        let y: string = x.a;
+>y : Symbol(y, Decl(typeGuardOfFromPropNameInUnionType.ts, 44, 11))
+>x.a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 42, 35))
+>a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+
+    } else {
+        let z: number = x.b;
+>z : Symbol(z, Decl(typeGuardOfFromPropNameInUnionType.ts, 46, 11))
+>x.b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 42, 35))
+>b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+    }
+}
+
+class ClassWithUnionProp { prop: A | B; }
+>ClassWithUnionProp : Symbol(ClassWithUnionProp, Decl(typeGuardOfFromPropNameInUnionType.ts, 48, 1))
+>prop : Symbol(ClassWithUnionProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 50, 26))
+>A : Symbol(A, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 0))
+>B : Symbol(B, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 22))
+
+function inProperty(x: ClassWithUnionProp) {
+>inProperty : Symbol(inProperty, Decl(typeGuardOfFromPropNameInUnionType.ts, 50, 41))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 52, 20))
+>ClassWithUnionProp : Symbol(ClassWithUnionProp, Decl(typeGuardOfFromPropNameInUnionType.ts, 48, 1))
+
+    if ("a" in x.prop) {
+>x.prop : Symbol(ClassWithUnionProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 50, 26))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 52, 20))
+>prop : Symbol(ClassWithUnionProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 50, 26))
+
+        let y: string = x.prop.a;
+>y : Symbol(y, Decl(typeGuardOfFromPropNameInUnionType.ts, 54, 11))
+>x.prop.a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+>x.prop : Symbol(ClassWithUnionProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 50, 26))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 52, 20))
+>prop : Symbol(ClassWithUnionProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 50, 26))
+>a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+
+    } else {
+        let z: number = x.prop.b;
+>z : Symbol(z, Decl(typeGuardOfFromPropNameInUnionType.ts, 56, 11))
+>x.prop.b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+>x.prop : Symbol(ClassWithUnionProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 50, 26))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 52, 20))
+>prop : Symbol(ClassWithUnionProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 50, 26))
+>b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+    }
+}
+
+class NestedClassWithProp { outer: ClassWithUnionProp; }
+>NestedClassWithProp : Symbol(NestedClassWithProp, Decl(typeGuardOfFromPropNameInUnionType.ts, 58, 1))
+>outer : Symbol(NestedClassWithProp.outer, Decl(typeGuardOfFromPropNameInUnionType.ts, 60, 27))
+>ClassWithUnionProp : Symbol(ClassWithUnionProp, Decl(typeGuardOfFromPropNameInUnionType.ts, 48, 1))
+
+function innestedProperty(x: NestedClassWithProp) {
+>innestedProperty : Symbol(innestedProperty, Decl(typeGuardOfFromPropNameInUnionType.ts, 60, 56))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 62, 26))
+>NestedClassWithProp : Symbol(NestedClassWithProp, Decl(typeGuardOfFromPropNameInUnionType.ts, 58, 1))
+
+    if ("a" in x.outer.prop) {
+>x.outer.prop : Symbol(ClassWithUnionProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 50, 26))
+>x.outer : Symbol(NestedClassWithProp.outer, Decl(typeGuardOfFromPropNameInUnionType.ts, 60, 27))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 62, 26))
+>outer : Symbol(NestedClassWithProp.outer, Decl(typeGuardOfFromPropNameInUnionType.ts, 60, 27))
+>prop : Symbol(ClassWithUnionProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 50, 26))
+
+        let y: string = x.outer.prop.a;
+>y : Symbol(y, Decl(typeGuardOfFromPropNameInUnionType.ts, 64, 11))
+>x.outer.prop.a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+>x.outer.prop : Symbol(ClassWithUnionProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 50, 26))
+>x.outer : Symbol(NestedClassWithProp.outer, Decl(typeGuardOfFromPropNameInUnionType.ts, 60, 27))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 62, 26))
+>outer : Symbol(NestedClassWithProp.outer, Decl(typeGuardOfFromPropNameInUnionType.ts, 60, 27))
+>prop : Symbol(ClassWithUnionProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 50, 26))
+>a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+
+    } else {
+        let z: number = x.outer.prop.b;
+>z : Symbol(z, Decl(typeGuardOfFromPropNameInUnionType.ts, 66, 11))
+>x.outer.prop.b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+>x.outer.prop : Symbol(ClassWithUnionProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 50, 26))
+>x.outer : Symbol(NestedClassWithProp.outer, Decl(typeGuardOfFromPropNameInUnionType.ts, 60, 27))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 62, 26))
+>outer : Symbol(NestedClassWithProp.outer, Decl(typeGuardOfFromPropNameInUnionType.ts, 60, 27))
+>prop : Symbol(ClassWithUnionProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 50, 26))
+>b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+    }
+}
+
+class InMemberOfClass {
+>InMemberOfClass : Symbol(InMemberOfClass, Decl(typeGuardOfFromPropNameInUnionType.ts, 68, 1))
+
+    protected prop: A | B;
+>prop : Symbol(InMemberOfClass.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 70, 23))
+>A : Symbol(A, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 0))
+>B : Symbol(B, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 22))
+
+    inThis() {
+>inThis : Symbol(InMemberOfClass.inThis, Decl(typeGuardOfFromPropNameInUnionType.ts, 71, 26))
+
+        if ("a" in this.prop) {
+>this.prop : Symbol(InMemberOfClass.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 70, 23))
+>this : Symbol(InMemberOfClass, Decl(typeGuardOfFromPropNameInUnionType.ts, 68, 1))
+>prop : Symbol(InMemberOfClass.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 70, 23))
+
+            let y: string = this.prop.a;
+>y : Symbol(y, Decl(typeGuardOfFromPropNameInUnionType.ts, 74, 15))
+>this.prop.a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+>this.prop : Symbol(InMemberOfClass.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 70, 23))
+>this : Symbol(InMemberOfClass, Decl(typeGuardOfFromPropNameInUnionType.ts, 68, 1))
+>prop : Symbol(InMemberOfClass.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 70, 23))
+>a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+
+        } else {
+            let z: number = this.prop.b;
+>z : Symbol(z, Decl(typeGuardOfFromPropNameInUnionType.ts, 76, 15))
+>this.prop.b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+>this.prop : Symbol(InMemberOfClass.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 70, 23))
+>this : Symbol(InMemberOfClass, Decl(typeGuardOfFromPropNameInUnionType.ts, 68, 1))
+>prop : Symbol(InMemberOfClass.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 70, 23))
+>b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+        }
+    }
+}
+
+// added for completeness
+class SelfAssert {
+>SelfAssert : Symbol(SelfAssert, Decl(typeGuardOfFromPropNameInUnionType.ts, 79, 1))
+
+    a: string;
+>a : Symbol(SelfAssert.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 82, 18))
+
+    inThis() {
+>inThis : Symbol(SelfAssert.inThis, Decl(typeGuardOfFromPropNameInUnionType.ts, 83, 14))
+
+        if ("a" in this) {
+>this : Symbol(SelfAssert, Decl(typeGuardOfFromPropNameInUnionType.ts, 79, 1))
+
+            let y: string = this.a;
+>y : Symbol(y, Decl(typeGuardOfFromPropNameInUnionType.ts, 86, 15))
+>this.a : Symbol(SelfAssert.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 82, 18))
+>this : Symbol(SelfAssert, Decl(typeGuardOfFromPropNameInUnionType.ts, 79, 1))
+>a : Symbol(SelfAssert.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 82, 18))
+
+        } else {
+        }
+    }
+}
diff --git a/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.types b/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.types
new file mode 100644
index 0000000000000..4aff12ae07759
--- /dev/null
+++ b/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.types
@@ -0,0 +1,318 @@
+=== tests/cases/conformance/expressions/typeGuards/typeGuardOfFromPropNameInUnionType.ts ===
+class A { a: string; }
+>A : A
+>a : string
+
+class B { b: number; }
+>B : B
+>b : number
+
+class C { b: Object; }
+>C : C
+>b : Object
+>Object : Object
+
+class D { a: Date; }
+>D : D
+>a : Date
+>Date : Date
+
+function namedClasses(x: A | B) {
+>namedClasses : (x: A | B) => void
+>x : A | B
+>A : A
+>B : B
+
+    if ("a" in x) {
+>"a" in x : boolean
+>"a" : "a"
+>x : A | B
+
+        x.a = "1";
+>x.a = "1" : "1"
+>x.a : string
+>x : A
+>a : string
+>"1" : "1"
+
+    } else {
+        x.b = 1;
+>x.b = 1 : 1
+>x.b : number
+>x : B
+>b : number
+>1 : 1
+    }
+}
+
+function multipleClasses(x: A | B | C | D) {
+>multipleClasses : (x: A | B | C | D) => void
+>x : A | B | C | D
+>A : A
+>B : B
+>C : C
+>D : D
+
+    if ("a" in x) {
+>"a" in x : boolean
+>"a" : "a"
+>x : A | B | C | D
+
+        let y: string | Date = x.a;
+>y : string | Date
+>Date : Date
+>x.a : string | Date
+>x : A | D
+>a : string | Date
+
+    } else {
+        let z: number | Object = x.b;
+>z : number | Object
+>Object : Object
+>x.b : number | Object
+>x : B | C
+>b : number | Object
+    }
+}
+
+function anonymousClasses(x: { a: string; } | { b: number; }) {
+>anonymousClasses : (x: { a: string; } | { b: number; }) => void
+>x : { a: string; } | { b: number; }
+>a : string
+>b : number
+
+    if ("a" in x) {
+>"a" in x : boolean
+>"a" : "a"
+>x : { a: string; } | { b: number; }
+
+        let y: string = x.a;
+>y : string
+>x.a : string
+>x : { a: string; }
+>a : string
+
+    } else {
+        let z: number = x.b;
+>z : number
+>x.b : number
+>x : { b: number; }
+>b : number
+    }
+}
+
+class AWithOptionalProp { a?: string; }
+>AWithOptionalProp : AWithOptionalProp
+>a : string
+
+class BWithOptionalProp { b?: string; }
+>BWithOptionalProp : BWithOptionalProp
+>b : string
+
+function positiveTestClassesWithOptionalProperties(x: AWithOptionalProp | BWithOptionalProp) {
+>positiveTestClassesWithOptionalProperties : (x: AWithOptionalProp | BWithOptionalProp) => void
+>x : AWithOptionalProp | BWithOptionalProp
+>AWithOptionalProp : AWithOptionalProp
+>BWithOptionalProp : BWithOptionalProp
+
+    if ("a" in x) {
+>"a" in x : boolean
+>"a" : "a"
+>x : AWithOptionalProp | BWithOptionalProp
+
+        x.a = "1";
+>x.a = "1" : "1"
+>x.a : string
+>x : AWithOptionalProp
+>a : string
+>"1" : "1"
+
+    } else {
+        const y: string = x instanceof AWithOptionalProp
+>y : string
+>x instanceof AWithOptionalProp            ? x.a            : x.b : string
+>x instanceof AWithOptionalProp : boolean
+>x : AWithOptionalProp | BWithOptionalProp
+>AWithOptionalProp : typeof AWithOptionalProp
+
+            ? x.a
+>x.a : string
+>x : AWithOptionalProp
+>a : string
+
+            : x.b
+>x.b : string
+>x : BWithOptionalProp
+>b : string
+    }
+}
+
+function inParenthesizedExpression(x: A | B) {
+>inParenthesizedExpression : (x: A | B) => void
+>x : A | B
+>A : A
+>B : B
+
+    if ("a" in (x)) {
+>"a" in (x) : boolean
+>"a" : "a"
+>(x) : A | B
+>x : A | B
+
+        let y: string = x.a;
+>y : string
+>x.a : string
+>x : A
+>a : string
+
+    } else {
+        let z: number = x.b;
+>z : number
+>x.b : number
+>x : B
+>b : number
+    }
+}
+
+class ClassWithUnionProp { prop: A | B; }
+>ClassWithUnionProp : ClassWithUnionProp
+>prop : A | B
+>A : A
+>B : B
+
+function inProperty(x: ClassWithUnionProp) {
+>inProperty : (x: ClassWithUnionProp) => void
+>x : ClassWithUnionProp
+>ClassWithUnionProp : ClassWithUnionProp
+
+    if ("a" in x.prop) {
+>"a" in x.prop : boolean
+>"a" : "a"
+>x.prop : A | B
+>x : ClassWithUnionProp
+>prop : A | B
+
+        let y: string = x.prop.a;
+>y : string
+>x.prop.a : string
+>x.prop : A
+>x : ClassWithUnionProp
+>prop : A
+>a : string
+
+    } else {
+        let z: number = x.prop.b;
+>z : number
+>x.prop.b : number
+>x.prop : B
+>x : ClassWithUnionProp
+>prop : B
+>b : number
+    }
+}
+
+class NestedClassWithProp { outer: ClassWithUnionProp; }
+>NestedClassWithProp : NestedClassWithProp
+>outer : ClassWithUnionProp
+>ClassWithUnionProp : ClassWithUnionProp
+
+function innestedProperty(x: NestedClassWithProp) {
+>innestedProperty : (x: NestedClassWithProp) => void
+>x : NestedClassWithProp
+>NestedClassWithProp : NestedClassWithProp
+
+    if ("a" in x.outer.prop) {
+>"a" in x.outer.prop : boolean
+>"a" : "a"
+>x.outer.prop : A | B
+>x.outer : ClassWithUnionProp
+>x : NestedClassWithProp
+>outer : ClassWithUnionProp
+>prop : A | B
+
+        let y: string = x.outer.prop.a;
+>y : string
+>x.outer.prop.a : string
+>x.outer.prop : A
+>x.outer : ClassWithUnionProp
+>x : NestedClassWithProp
+>outer : ClassWithUnionProp
+>prop : A
+>a : string
+
+    } else {
+        let z: number = x.outer.prop.b;
+>z : number
+>x.outer.prop.b : number
+>x.outer.prop : B
+>x.outer : ClassWithUnionProp
+>x : NestedClassWithProp
+>outer : ClassWithUnionProp
+>prop : B
+>b : number
+    }
+}
+
+class InMemberOfClass {
+>InMemberOfClass : InMemberOfClass
+
+    protected prop: A | B;
+>prop : A | B
+>A : A
+>B : B
+
+    inThis() {
+>inThis : () => void
+
+        if ("a" in this.prop) {
+>"a" in this.prop : boolean
+>"a" : "a"
+>this.prop : A | B
+>this : this
+>prop : A | B
+
+            let y: string = this.prop.a;
+>y : string
+>this.prop.a : string
+>this.prop : A
+>this : this
+>prop : A
+>a : string
+
+        } else {
+            let z: number = this.prop.b;
+>z : number
+>this.prop.b : number
+>this.prop : B
+>this : this
+>prop : B
+>b : number
+        }
+    }
+}
+
+// added for completeness
+class SelfAssert {
+>SelfAssert : SelfAssert
+
+    a: string;
+>a : string
+
+    inThis() {
+>inThis : () => void
+
+        if ("a" in this) {
+>"a" in this : boolean
+>"a" : "a"
+>this : this
+
+            let y: string = this.a;
+>y : string
+>this.a : string
+>this : this
+>a : string
+
+        } else {
+        }
+    }
+}
diff --git a/tests/cases/compiler/inKeywordTypeguard.ts b/tests/cases/compiler/inKeywordTypeguard.ts
new file mode 100644
index 0000000000000..bbab20afc6f41
--- /dev/null
+++ b/tests/cases/compiler/inKeywordTypeguard.ts
@@ -0,0 +1,97 @@
+class A { a: string; }
+class B { b: string; }
+
+function negativeClassesTest(x: A | B) {
+    if ("a" in x) {
+        x.b = "1";
+    } else {
+        x.a = "1";
+    }
+}
+
+function positiveClassesTest(x: A | B) {
+    if ("a" in x) {
+        x.b = "1";
+    } else {
+        x.a = "1";
+    }
+}
+
+class AWithOptionalProp { a?: string; }
+class BWithOptionalProp { b?: string; }
+
+function positiveTestClassesWithOptionalProperties(x: AWithOptionalProp | BWithOptionalProp) {
+    if ("a" in x) {
+        x.a = "1";
+    } else {
+        x.b = "1";
+    }
+}
+
+class AWithMethod {
+    a(): string { return ""; }
+}
+
+class BWithMethod {
+    b(): string { return ""; }
+}
+
+function negativeTestClassesWithMembers(x: AWithMethod | BWithMethod) {
+    if ("a" in x) {
+        x.a();
+        x.b();
+    } else {
+    }
+}
+
+function negativeTestClassesWithMemberMissingInBothClasses(x: AWithMethod | BWithMethod) {
+    if ("c" in x) {
+        x.a();
+        x.b();
+    } else {
+        x.a();
+        x.b();
+    }
+}
+
+class C { a: string; }
+class D { a: string; }
+
+function negativeMultipleClassesTest(x: A | B | C | D) {
+    if ("a" in x) {
+        x.b = "1";
+    } else {
+        x.a = "1";
+    }
+}
+
+class ClassWithUnionProp { prop: A | B }
+
+function negativePropTest(x: ClassWithUnionProp) {
+    if ("a" in x.prop) {
+        let y: string = x.prop.b;
+    } else {
+        let z: string = x.prop.a;
+    }
+}
+
+class NegativeClassTest {
+    protected prop: A | B;
+    inThis() {
+        if ("a" in this.prop) {
+            let z: number = this.prop.b;
+        } else {
+            let y: string = this.prop.a;
+        }
+    }
+}
+
+class UnreachableCodeDetection {
+    a: string;
+    inThis() {
+        if ("a" in this) {
+        } else {
+            let y = this.a;
+        }
+    }
+}
\ No newline at end of file
diff --git a/tests/cases/conformance/expressions/typeGuards/typeGuardOfFromPropNameInUnionType.ts b/tests/cases/conformance/expressions/typeGuards/typeGuardOfFromPropNameInUnionType.ts
new file mode 100644
index 0000000000000..d89d7b8c4efcb
--- /dev/null
+++ b/tests/cases/conformance/expressions/typeGuards/typeGuardOfFromPropNameInUnionType.ts
@@ -0,0 +1,91 @@
+class A { a: string; }
+class B { b: number; }
+class C { b: Object; }
+class D { a: Date; }
+
+function namedClasses(x: A | B) {
+    if ("a" in x) {
+        x.a = "1";
+    } else {
+        x.b = 1;
+    }
+}
+
+function multipleClasses(x: A | B | C | D) {
+    if ("a" in x) {
+        let y: string | Date = x.a;
+    } else {
+        let z: number | Object = x.b;
+    }
+}
+
+function anonymousClasses(x: { a: string; } | { b: number; }) {
+    if ("a" in x) {
+        let y: string = x.a;
+    } else {
+        let z: number = x.b;
+    }
+}
+
+class AWithOptionalProp { a?: string; }
+class BWithOptionalProp { b?: string; }
+
+function positiveTestClassesWithOptionalProperties(x: AWithOptionalProp | BWithOptionalProp) {
+    if ("a" in x) {
+        x.a = "1";
+    } else {
+        const y: string = x instanceof AWithOptionalProp
+            ? x.a
+            : x.b
+    }
+}
+
+function inParenthesizedExpression(x: A | B) {
+    if ("a" in (x)) {
+        let y: string = x.a;
+    } else {
+        let z: number = x.b;
+    }
+}
+
+class ClassWithUnionProp { prop: A | B; }
+
+function inProperty(x: ClassWithUnionProp) {
+    if ("a" in x.prop) {
+        let y: string = x.prop.a;
+    } else {
+        let z: number = x.prop.b;
+    }
+}
+
+class NestedClassWithProp { outer: ClassWithUnionProp; }
+
+function innestedProperty(x: NestedClassWithProp) {
+    if ("a" in x.outer.prop) {
+        let y: string = x.outer.prop.a;
+    } else {
+        let z: number = x.outer.prop.b;
+    }
+}
+
+class InMemberOfClass {
+    protected prop: A | B;
+    inThis() {
+        if ("a" in this.prop) {
+            let y: string = this.prop.a;
+        } else {
+            let z: number = this.prop.b;
+        }
+    }
+}
+
+// added for completeness
+class SelfAssert {
+    a: string;
+    inThis() {
+        if ("a" in this) {
+            let y: string = this.a;
+        } else {
+        }
+    }
+}
\ No newline at end of file