Add depth limiter to isConstTypeVariable function (#54624)

Author: ahejlsberg

src/compiler/checker.ts

@@ -13538,14 +13538,14 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         return hasNonCircularBaseConstraint(typeParameter) ? getConstraintFromTypeParameter(typeParameter) : undefined;
     }
 
-    function isConstTypeVariable(type: Type | undefined): boolean {
-        return !!(type && (
+    function isConstTypeVariable(type: Type | undefined, depth = 0): boolean {
+        return depth < 5 && !!(type && (
             type.flags & TypeFlags.TypeParameter && some((type as TypeParameter).symbol?.declarations, d => hasSyntacticModifier(d, ModifierFlags.Const)) ||
-            type.flags & TypeFlags.Union && some((type as UnionType).types, isConstTypeVariable) ||
-            type.flags & TypeFlags.IndexedAccess && isConstTypeVariable((type as IndexedAccessType).objectType) ||
-            type.flags & TypeFlags.Conditional && isConstTypeVariable(getConstraintOfConditionalType(type as ConditionalType)) ||
-            type.flags & TypeFlags.Substitution && isConstTypeVariable((type as SubstitutionType).baseType) ||
-            isGenericTupleType(type) && findIndex(getElementTypes(type), (t, i) => !!(type.target.elementFlags[i] & ElementFlags.Variadic) && isConstTypeVariable(t)) >= 0));
+            type.flags & TypeFlags.Union && some((type as UnionType).types, t => isConstTypeVariable(t, depth)) ||
+            type.flags & TypeFlags.IndexedAccess && isConstTypeVariable((type as IndexedAccessType).objectType, depth + 1) ||
+            type.flags & TypeFlags.Conditional && isConstTypeVariable(getConstraintOfConditionalType(type as ConditionalType), depth + 1) ||
+            type.flags & TypeFlags.Substitution && isConstTypeVariable((type as SubstitutionType).baseType, depth) ||
+            isGenericTupleType(type) && findIndex(getElementTypes(type), (t, i) => !!(type.target.elementFlags[i] & ElementFlags.Variadic) && isConstTypeVariable(t, depth)) >= 0));
     }
 
     function getConstraintOfIndexedAccess(type: IndexedAccessType) {

tests/baselines/reference/circularBaseConstraint.errors.txt

@@ -0,0 +1,22 @@
+circularBaseConstraint.ts(14,8): error TS2304: Cannot find name 'a'.
+
+
+==== circularBaseConstraint.ts (1 errors) ====
+    // Repro from #54610
+    
+    type A<T> = T;
+    
+    type B<T> = T extends any[]
+        ? never
+        : A<T> extends infer key
+        ? key extends keyof T
+            ? B<T[key]>
+            : never
+        : never;
+    
+    function foo<T>() {
+        `${a}` as B<T>;
+           ~
+!!! error TS2304: Cannot find name 'a'.
+    }
+    

tests/baselines/reference/circularBaseConstraint.symbols

@@ -0,0 +1,42 @@
+//// [tests/cases/compiler/circularBaseConstraint.ts] ////
+
+=== circularBaseConstraint.ts ===
+// Repro from #54610
+
+type A<T> = T;
+>A : Symbol(A, Decl(circularBaseConstraint.ts, 0, 0))
+>T : Symbol(T, Decl(circularBaseConstraint.ts, 2, 7))
+>T : Symbol(T, Decl(circularBaseConstraint.ts, 2, 7))
+
+type B<T> = T extends any[]
+>B : Symbol(B, Decl(circularBaseConstraint.ts, 2, 14))
+>T : Symbol(T, Decl(circularBaseConstraint.ts, 4, 7))
+>T : Symbol(T, Decl(circularBaseConstraint.ts, 4, 7))
+
+    ? never
+    : A<T> extends infer key
+>A : Symbol(A, Decl(circularBaseConstraint.ts, 0, 0))
+>T : Symbol(T, Decl(circularBaseConstraint.ts, 4, 7))
+>key : Symbol(key, Decl(circularBaseConstraint.ts, 6, 24))
+
+    ? key extends keyof T
+>key : Symbol(key, Decl(circularBaseConstraint.ts, 6, 24))
+>T : Symbol(T, Decl(circularBaseConstraint.ts, 4, 7))
+
+        ? B<T[key]>
+>B : Symbol(B, Decl(circularBaseConstraint.ts, 2, 14))
+>T : Symbol(T, Decl(circularBaseConstraint.ts, 4, 7))
+>key : Symbol(key, Decl(circularBaseConstraint.ts, 6, 24))
+
+        : never
+    : never;
+
+function foo<T>() {
+>foo : Symbol(foo, Decl(circularBaseConstraint.ts, 10, 12))
+>T : Symbol(T, Decl(circularBaseConstraint.ts, 12, 13))
+
+    `${a}` as B<T>;
+>B : Symbol(B, Decl(circularBaseConstraint.ts, 2, 14))
+>T : Symbol(T, Decl(circularBaseConstraint.ts, 12, 13))
+}
+

tests/baselines/reference/circularBaseConstraint.types

@@ -0,0 +1,27 @@
+//// [tests/cases/compiler/circularBaseConstraint.ts] ////
+
+=== circularBaseConstraint.ts ===
+// Repro from #54610
+
+type A<T> = T;
+>A : T
+
+type B<T> = T extends any[]
+>B : B<T>
+
+    ? never
+    : A<T> extends infer key
+    ? key extends keyof T
+        ? B<T[key]>
+        : never
+    : never;
+
+function foo<T>() {
+>foo : <T>() => void
+
+    `${a}` as B<T>;
+>`${a}` as B<T> : B<T>
+>`${a}` : string
+>a : any
+}
+

tests/cases/compiler/circularBaseConstraint.ts

@@ -0,0 +1,18 @@
+// @strict: true
+// @noEmit: true
+
+// Repro from #54610
+
+type A<T> = T;
+
+type B<T> = T extends any[]
+    ? never
+    : A<T> extends infer key
+    ? key extends keyof T
+        ? B<T[key]>
+        : never
+    : never;
+
+function foo<T>() {
+    `${a}` as B<T>;
+}