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Fix type parameter leak when using 'this' in reverse mapped types
Fixes #62779 When object literal methods reference `this` inside a function with a reverse mapped type parameter, the type parameter T was leaking through. For example: ```typescript declare function test<T extends Record<string, unknown>>(obj: { [K in keyof T]: () => T[K]; }): T; const obj = test({ a() { return 0; }, b() { return this.a(); }, }); // Was: { a: number; b: T[string]; } (widened to unknown) // Now: { a: number; b: number; } ``` The fix modifies `getContextualThisParameterType` to use the actual object literal type when in an inference context and the contextual type contains a mapped type. This allows methods to see each other's already-inferred types when resolving `this` references. This also handles intersection and union types containing mapped types.
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‎src/compiler/checker.ts‎

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@@ -31825,6 +31825,13 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
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// There was no contextual ThisType<T> for the containing object literal, so the contextual type
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// for 'this' is the non-null form of the contextual type for the containing object literal or
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// the type of the object literal itself.
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// If we're in an inference context and the contextual type contains a mapped type (like reverse
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// mapped types used for inference), use the object literal type to allow proper inference
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// of method return types that reference other methods via this. This avoids leaking type
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// parameters like T[K] when methods call each other.
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if (contextualType && someType(contextualType, t => !!(getObjectFlags(t) & ObjectFlags.Mapped)) && getInferenceContext(containingLiteral)) {
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return getWidenedType(checkExpressionCached(containingLiteral));
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}
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return getWidenedType(contextualType ? getNonNullableType(contextualType) : checkExpressionCached(containingLiteral));
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}
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// In an assignment of the form 'obj.xxx = function(...)' or 'obj[xxx] = function(...)', the

‎tests/baselines/reference/reverseMappedThisTypeInference.js‎

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//// [tests/cases/compiler/reverseMappedThisTypeInference.ts] ////
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//// [reverseMappedThisTypeInference.ts]
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// Issue #62779: Type parameter leak caused by `this` and reverse mapped type
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declare function testReverseMapped<T extends Record<string, unknown>>(obj: {
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[K in keyof T]: () => T[K];
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}): T;
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const obj = testReverseMapped({
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a() {
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return 0;
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},
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b() {
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return this.a();
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},
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});
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// Intersection with mapped type
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declare function testReverseMapped2<T extends Record<string, unknown>, T2>(
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obj: T2 & {
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[K in keyof T]: () => T[K];
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},
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): T;
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const obj2 = testReverseMapped2({
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a() {
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return 0;
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},
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b() {
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return this.a();
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},
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});
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// Union with mapped type
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declare function testReverseMapped3<T extends Record<string, unknown>, T2>(
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obj: T2 | {
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[K in keyof T]: () => T[K];
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},
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): T;
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const obj3 = testReverseMapped3({
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a() {
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return 0;
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},
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b() {
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return this.a();
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},
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});
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//// [reverseMappedThisTypeInference.js]
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"use strict";
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var obj = testReverseMapped({
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a: function () {
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return 0;
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},
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b: function () {
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return this.a();
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},
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});
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var obj2 = testReverseMapped2({
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a: function () {
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return 0;
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},
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b: function () {
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return this.a();
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},
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});
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var obj3 = testReverseMapped3({
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a: function () {
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return 0;
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},
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b: function () {
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return this.a();
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},
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});

‎tests/baselines/reference/reverseMappedThisTypeInference.symbols‎

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//// [tests/cases/compiler/reverseMappedThisTypeInference.ts] ////
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=== reverseMappedThisTypeInference.ts ===
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// Issue #62779: Type parameter leak caused by `this` and reverse mapped type
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declare function testReverseMapped<T extends Record<string, unknown>>(obj: {
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>testReverseMapped : Symbol(testReverseMapped, Decl(reverseMappedThisTypeInference.ts, 0, 0))
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>T : Symbol(T, Decl(reverseMappedThisTypeInference.ts, 1, 35))
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>Record : Symbol(Record, Decl(lib.es5.d.ts, --, --))
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>obj : Symbol(obj, Decl(reverseMappedThisTypeInference.ts, 1, 70))
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[K in keyof T]: () => T[K];
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>K : Symbol(K, Decl(reverseMappedThisTypeInference.ts, 2, 5))
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>T : Symbol(T, Decl(reverseMappedThisTypeInference.ts, 1, 35))
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>T : Symbol(T, Decl(reverseMappedThisTypeInference.ts, 1, 35))
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>K : Symbol(K, Decl(reverseMappedThisTypeInference.ts, 2, 5))
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}): T;
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>T : Symbol(T, Decl(reverseMappedThisTypeInference.ts, 1, 35))
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const obj = testReverseMapped({
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>obj : Symbol(obj, Decl(reverseMappedThisTypeInference.ts, 5, 5))
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>testReverseMapped : Symbol(testReverseMapped, Decl(reverseMappedThisTypeInference.ts, 0, 0))
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a() {
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>a : Symbol(a, Decl(reverseMappedThisTypeInference.ts, 5, 31))
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return 0;
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},
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b() {
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>b : Symbol(b, Decl(reverseMappedThisTypeInference.ts, 8, 6))
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return this.a();
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>this.a : Symbol(a, Decl(reverseMappedThisTypeInference.ts, 5, 31))
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>this : Symbol(__type, Decl(reverseMappedThisTypeInference.ts, 1, 74))
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>a : Symbol(a, Decl(reverseMappedThisTypeInference.ts, 5, 31))
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},
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});
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// Intersection with mapped type
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declare function testReverseMapped2<T extends Record<string, unknown>, T2>(
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>testReverseMapped2 : Symbol(testReverseMapped2, Decl(reverseMappedThisTypeInference.ts, 12, 3))
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>T : Symbol(T, Decl(reverseMappedThisTypeInference.ts, 15, 36))
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>Record : Symbol(Record, Decl(lib.es5.d.ts, --, --))
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>T2 : Symbol(T2, Decl(reverseMappedThisTypeInference.ts, 15, 70))
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obj: T2 & {
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>obj : Symbol(obj, Decl(reverseMappedThisTypeInference.ts, 15, 75))
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>T2 : Symbol(T2, Decl(reverseMappedThisTypeInference.ts, 15, 70))
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[K in keyof T]: () => T[K];
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>K : Symbol(K, Decl(reverseMappedThisTypeInference.ts, 17, 9))
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>T : Symbol(T, Decl(reverseMappedThisTypeInference.ts, 15, 36))
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>T : Symbol(T, Decl(reverseMappedThisTypeInference.ts, 15, 36))
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>K : Symbol(K, Decl(reverseMappedThisTypeInference.ts, 17, 9))
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},
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): T;
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>T : Symbol(T, Decl(reverseMappedThisTypeInference.ts, 15, 36))
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const obj2 = testReverseMapped2({
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>obj2 : Symbol(obj2, Decl(reverseMappedThisTypeInference.ts, 21, 5))
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>testReverseMapped2 : Symbol(testReverseMapped2, Decl(reverseMappedThisTypeInference.ts, 12, 3))
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a() {
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>a : Symbol(a, Decl(reverseMappedThisTypeInference.ts, 21, 33))
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return 0;
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},
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b() {
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>b : Symbol(b, Decl(reverseMappedThisTypeInference.ts, 24, 6))
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return this.a();
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>this.a : Symbol(a, Decl(reverseMappedThisTypeInference.ts, 21, 33), Decl(reverseMappedThisTypeInference.ts, 21, 33))
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>a : Symbol(a, Decl(reverseMappedThisTypeInference.ts, 21, 33), Decl(reverseMappedThisTypeInference.ts, 21, 33))
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},
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});
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// Union with mapped type
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declare function testReverseMapped3<T extends Record<string, unknown>, T2>(
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>testReverseMapped3 : Symbol(testReverseMapped3, Decl(reverseMappedThisTypeInference.ts, 28, 3))
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>T : Symbol(T, Decl(reverseMappedThisTypeInference.ts, 31, 36))
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>Record : Symbol(Record, Decl(lib.es5.d.ts, --, --))
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>T2 : Symbol(T2, Decl(reverseMappedThisTypeInference.ts, 31, 70))
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obj: T2 | {
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>obj : Symbol(obj, Decl(reverseMappedThisTypeInference.ts, 31, 75))
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>T2 : Symbol(T2, Decl(reverseMappedThisTypeInference.ts, 31, 70))
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[K in keyof T]: () => T[K];
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>K : Symbol(K, Decl(reverseMappedThisTypeInference.ts, 33, 9))
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>T : Symbol(T, Decl(reverseMappedThisTypeInference.ts, 31, 36))
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>T : Symbol(T, Decl(reverseMappedThisTypeInference.ts, 31, 36))
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>K : Symbol(K, Decl(reverseMappedThisTypeInference.ts, 33, 9))
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},
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): T;
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>T : Symbol(T, Decl(reverseMappedThisTypeInference.ts, 31, 36))
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const obj3 = testReverseMapped3({
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>obj3 : Symbol(obj3, Decl(reverseMappedThisTypeInference.ts, 37, 5))
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>testReverseMapped3 : Symbol(testReverseMapped3, Decl(reverseMappedThisTypeInference.ts, 28, 3))
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a() {
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>a : Symbol(a, Decl(reverseMappedThisTypeInference.ts, 37, 33))
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return 0;
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},
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b() {
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>b : Symbol(b, Decl(reverseMappedThisTypeInference.ts, 40, 6))
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return this.a();
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>this.a : Symbol(a, Decl(reverseMappedThisTypeInference.ts, 37, 33), Decl(reverseMappedThisTypeInference.ts, 37, 33))
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>a : Symbol(a, Decl(reverseMappedThisTypeInference.ts, 37, 33), Decl(reverseMappedThisTypeInference.ts, 37, 33))
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},
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});
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‎tests/baselines/reference/reverseMappedThisTypeInference.types‎

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//// [tests/cases/compiler/reverseMappedThisTypeInference.ts] ////
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=== reverseMappedThisTypeInference.ts ===
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// Issue #62779: Type parameter leak caused by `this` and reverse mapped type
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declare function testReverseMapped<T extends Record<string, unknown>>(obj: {
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>testReverseMapped : <T extends Record<string, unknown>>(obj: { [K in keyof T]: () => T[K]; }) => T
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> : ^ ^^^^^^^^^ ^^ ^^ ^^^^^
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>obj : { [K in keyof T]: () => T[K]; }
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> : ^^^ ^^^^^^^^^^^^^^^^^^^^ ^^^
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[K in keyof T]: () => T[K];
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}): T;
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const obj = testReverseMapped({
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>obj : { a: number; b: number; }
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^
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>testReverseMapped({ a() { return 0; }, b() { return this.a(); },}) : { a: number; b: number; }
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^
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>testReverseMapped : <T extends Record<string, unknown>>(obj: { [K in keyof T]: () => T[K]; }) => T
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> : ^ ^^^^^^^^^ ^^ ^^ ^^^^^
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>{ a() { return 0; }, b() { return this.a(); },} : { a(): number; b(): number; }
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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a() {
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>a : () => number
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> : ^^^^^^^^^^^^
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return 0;
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>0 : 0
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> : ^
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},
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b() {
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>b : () => number
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> : ^^^^^^^^^^^^
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return this.a();
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>this.a() : number
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> : ^^^^^^
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>this.a : () => number
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> : ^^^^^^^^^^^^
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>this : { a: () => number; b: () => number; }
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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>a : () => number
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> : ^^^^^^^^^^^^
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},
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});
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// Intersection with mapped type
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declare function testReverseMapped2<T extends Record<string, unknown>, T2>(
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>testReverseMapped2 : <T extends Record<string, unknown>, T2>(obj: T2 & { [K in keyof T]: () => T[K]; }) => T
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> : ^ ^^^^^^^^^ ^^ ^^ ^^ ^^^^^
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obj: T2 & {
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>obj : T2 & { [K in keyof T]: () => T[K]; }
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> : ^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^ ^^^
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[K in keyof T]: () => T[K];
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},
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): T;
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const obj2 = testReverseMapped2({
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>obj2 : { a: number; b: number; }
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^
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>testReverseMapped2({ a() { return 0; }, b() { return this.a(); },}) : { a: number; b: number; }
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^
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>testReverseMapped2 : <T extends Record<string, unknown>, T2>(obj: T2 & { [K in keyof T]: () => T[K]; }) => T
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> : ^ ^^^^^^^^^ ^^ ^^ ^^ ^^^^^
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>{ a() { return 0; }, b() { return this.a(); },} : { a(): number; b(): number; }
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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a() {
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>a : () => number
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> : ^^^^^^^^^^^^
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return 0;
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>0 : 0
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> : ^
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},
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b() {
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>b : () => number
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> : ^^^^^^^^^^^^
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return this.a();
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>this.a() : number
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> : ^^^^^^
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>this.a : (() => number) & (() => number)
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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>this : { a(): number; b(): number; } & { a: () => number; b: () => number; }
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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>a : (() => number) & (() => number)
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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},
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});
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// Union with mapped type
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declare function testReverseMapped3<T extends Record<string, unknown>, T2>(
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>testReverseMapped3 : <T extends Record<string, unknown>, T2>(obj: T2 | { [K in keyof T]: () => T[K]; }) => T
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> : ^ ^^^^^^^^^ ^^ ^^ ^^ ^^^^^
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obj: T2 | {
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>obj : T2 | { [K in keyof T]: () => T[K]; }
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> : ^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^ ^^^
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[K in keyof T]: () => T[K];
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},
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): T;
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const obj3 = testReverseMapped3({
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>obj3 : { a: number; b: number; }
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^
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>testReverseMapped3({ a() { return 0; }, b() { return this.a(); },}) : { a: number; b: number; }
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^
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>testReverseMapped3 : <T extends Record<string, unknown>, T2>(obj: T2 | { [K in keyof T]: () => T[K]; }) => T
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> : ^ ^^^^^^^^^ ^^ ^^ ^^ ^^^^^
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>{ a() { return 0; }, b() { return this.a(); },} : { a(): number; b(): number; }
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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a() {
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>a : () => number
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> : ^^^^^^^^^^^^
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return 0;
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>0 : 0
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> : ^
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},
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b() {
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>b : () => number
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> : ^^^^^^^^^^^^
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return this.a();
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>this.a() : number
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> : ^^^^^^
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>this.a : (() => number) | (() => number)
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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>this : { a(): number; b(): number; } | { a: () => number; b: () => number; }
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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>a : (() => number) | (() => number)
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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},
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});
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