Avoid infinite recursion when instantiating circular inline mapped generic tuple type (#53522)

Author: Andarist

src/compiler/checker.ts

@@ -18926,6 +18926,11 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             const singleton = elementFlags[i] & ElementFlags.Variadic ? t :
                 elementFlags[i] & ElementFlags.Rest ? createArrayType(t) :
                 createTupleType([t], [elementFlags[i]]);
+            // avoid infinite recursion, if the singleton is the type variable itself
+            // then we'd just get back here with the same arguments from within instantiateMappedType
+            if (singleton === typeVariable) {
+                return mappedType;
+            }
             // The singleton is never a generic tuple type, so it is safe to recurse here.
             return instantiateMappedType(mappedType, prependTypeMapping(typeVariable, singleton, mapper));
         });

tests/baselines/reference/circularInlineMappedGenericTupleTypeNoCrash.symbols

@@ -0,0 +1,45 @@
+=== tests/cases/compiler/circularInlineMappedGenericTupleTypeNoCrash.ts ===
+class Foo<Elements extends readonly unknown[]> {
+>Foo : Symbol(Foo, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 0))
+>Elements : Symbol(Elements, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 10))
+
+  public readonly elements: { [P in keyof Elements]: { bar: Elements[P] } };
+>elements : Symbol(Foo.elements, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 48))
+>P : Symbol(P, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 1, 31))
+>Elements : Symbol(Elements, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 10))
+>bar : Symbol(bar, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 1, 54))
+>Elements : Symbol(Elements, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 10))
+>P : Symbol(P, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 1, 31))
+
+  public constructor(
+    ...elements: { [P in keyof Elements]: { bar: Elements[P] } }
+>elements : Symbol(elements, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 3, 21))
+>P : Symbol(P, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 4, 20))
+>Elements : Symbol(Elements, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 10))
+>bar : Symbol(bar, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 4, 43))
+>Elements : Symbol(Elements, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 10))
+>P : Symbol(P, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 4, 20))
+
+  ) {
+    this.elements = elements;
+>this.elements : Symbol(Foo.elements, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 48))
+>this : Symbol(Foo, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 0))
+>elements : Symbol(Foo.elements, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 48))
+>elements : Symbol(elements, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 3, 21))
+  }
+
+  public add(): Foo<[...Elements, "abc"]> {
+>add : Symbol(Foo.add, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 7, 3))
+>Foo : Symbol(Foo, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 0))
+>Elements : Symbol(Elements, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 10))
+
+    return new Foo<[...Elements, "abc"]>(...this.elements, { bar: "abc" });
+>Foo : Symbol(Foo, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 0))
+>Elements : Symbol(Elements, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 10))
+>this.elements : Symbol(Foo.elements, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 48))
+>this : Symbol(Foo, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 0))
+>elements : Symbol(Foo.elements, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 0, 48))
+>bar : Symbol(bar, Decl(circularInlineMappedGenericTupleTypeNoCrash.ts, 10, 60))
+  }
+}
+

tests/baselines/reference/circularInlineMappedGenericTupleTypeNoCrash.types

@@ -0,0 +1,38 @@
+=== tests/cases/compiler/circularInlineMappedGenericTupleTypeNoCrash.ts ===
+class Foo<Elements extends readonly unknown[]> {
+>Foo : Foo<Elements>
+
+  public readonly elements: { [P in keyof Elements]: { bar: Elements[P] } };
+>elements : { [P in keyof Elements]: { bar: Elements[P]; }; }
+>bar : Elements[P]
+
+  public constructor(
+    ...elements: { [P in keyof Elements]: { bar: Elements[P] } }
+>elements : { [P in keyof Elements]: { bar: Elements[P]; }; }
+>bar : Elements[P]
+
+  ) {
+    this.elements = elements;
+>this.elements = elements : { [P in keyof Elements]: { bar: Elements[P]; }; }
+>this.elements : { [P in keyof Elements]: { bar: Elements[P]; }; }
+>this : this
+>elements : { [P in keyof Elements]: { bar: Elements[P]; }; }
+>elements : { [P in keyof Elements]: { bar: Elements[P]; }; }
+  }
+
+  public add(): Foo<[...Elements, "abc"]> {
+>add : () => Foo<[...Elements, "abc"]>
+
+    return new Foo<[...Elements, "abc"]>(...this.elements, { bar: "abc" });
+>new Foo<[...Elements, "abc"]>(...this.elements, { bar: "abc" }) : Foo<[...Elements, "abc"]>
+>Foo : typeof Foo
+>...this.elements : { bar: unknown; }
+>this.elements : { [P in keyof Elements]: { bar: Elements[P]; }; }
+>this : this
+>elements : { [P in keyof Elements]: { bar: Elements[P]; }; }
+>{ bar: "abc" } : { bar: "abc"; }
+>bar : "abc"
+>"abc" : "abc"
+  }
+}
+

tests/cases/compiler/circularInlineMappedGenericTupleTypeNoCrash.ts

@@ -0,0 +1,16 @@
+// @strict: true
+// @noEmit: true
+
+class Foo<Elements extends readonly unknown[]> {
+  public readonly elements: { [P in keyof Elements]: { bar: Elements[P] } };
+
+  public constructor(
+    ...elements: { [P in keyof Elements]: { bar: Elements[P] } }
+  ) {
+    this.elements = elements;
+  }
+
+  public add(): Foo<[...Elements, "abc"]> {
+    return new Foo<[...Elements, "abc"]>(...this.elements, { bar: "abc" });
+  }
+}