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Keep returned (and yielded) literal types as const when their types using const type variables #56859

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Keep returned (and yielded) literal types as const when their types using const type variables #56859

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6ad94f1
Keep returned (and yielded) literal types as const when their types u…
Andarist Dec 23, 2023
f057c90
add an extra test case with reverse mapped types
Andarist Dec 24, 2023
f3e57d6
Merge remote-tracking branch 'origin/main' into fix/returned-literals…
Andarist Jan 3, 2024
78831c2
address review feedback
Andarist Jan 3, 2024
b43558e
Merge remote-tracking branch 'origin/main' into fix/returned-literals…
Andarist Aug 9, 2025
913d009
add test
Andarist Aug 9, 2025
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7 changes: 5 additions & 2 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -36825,6 +36825,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
let fallbackReturnType: Type = voidType;
if (func.body.kind !== SyntaxKind.Block) { // Async or normal arrow function
returnType = checkExpressionCached(func.body, checkMode && checkMode & ~CheckMode.SkipGenericFunctions);
if (isConstContext(func.body)) {
returnType = getRegularTypeOfLiteralType(returnType);
}
if (isAsync) {
// From within an async function you can return either a non-promise value or a promise. Any
// Promise/A+ compatible implementation will always assimilate any foreign promise, so the
Expand Down Expand Up @@ -36952,7 +36955,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
const isAsync = (getFunctionFlags(func) & FunctionFlags.Async) !== 0;
forEachYieldExpression(func.body as Block, yieldExpression => {
const yieldExpressionType = yieldExpression.expression ? checkExpression(yieldExpression.expression, checkMode) : undefinedWideningType;
pushIfUnique(yieldTypes, getYieldedTypeOfYieldExpression(yieldExpression, yieldExpressionType, anyType, isAsync));
pushIfUnique(yieldTypes, getYieldedTypeOfYieldExpression(yieldExpression, yieldExpression.expression && isConstContext(yieldExpression.expression) ? getRegularTypeOfLiteralType(yieldExpressionType) : yieldExpressionType, anyType, isAsync));
let nextType: Type | undefined;
if (yieldExpression.asteriskToken) {
const iterationTypes = getIterationTypesOfIterable(
Expand Down Expand Up @@ -37074,7 +37077,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
if (type.flags & TypeFlags.Never) {
hasReturnOfTypeNever = true;
}
pushIfUnique(aggregatedTypes, type);
pushIfUnique(aggregatedTypes, isConstContext(expr) ? getRegularTypeOfLiteralType(type) : type);
}
else {
hasReturnWithNoExpression = true;
Expand Down
202 changes: 202 additions & 0 deletions tests/baselines/reference/typeParameterConstModifiersReturnsAndYields.symbols
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Original file line number Diff line number Diff line change
@@ -0,0 +1,202 @@
//// [tests/cases/conformance/types/typeParameters/typeParameterLists/typeParameterConstModifiersReturnsAndYields.ts] ////

=== typeParameterConstModifiersReturnsAndYields.ts ===
enum E { Val, Val2 }
>E : Symbol(E, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 0))
>Val : Symbol(E.Val, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 8))
>Val2 : Symbol(E.Val2, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 13))

declare function test1<const T>(create: () => T): T;
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))
>T : Symbol(T, Decl(typeParameterConstModifiersReturnsAndYields.ts, 2, 23))
>create : Symbol(create, Decl(typeParameterConstModifiersReturnsAndYields.ts, 2, 32))
>T : Symbol(T, Decl(typeParameterConstModifiersReturnsAndYields.ts, 2, 23))
>T : Symbol(T, Decl(typeParameterConstModifiersReturnsAndYields.ts, 2, 23))

const result1 = test1(() => ['a']);
>result1 : Symbol(result1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 4, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))

const result2 = test1(() => `a${Math.random()}`);
>result2 : Symbol(result2, Decl(typeParameterConstModifiersReturnsAndYields.ts, 5, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))
>Math.random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
>Math : Symbol(Math, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
>random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))

const result3 = test1(() => 'a');
>result3 : Symbol(result3, Decl(typeParameterConstModifiersReturnsAndYields.ts, 6, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))

const result4 = test1(() => true);
>result4 : Symbol(result4, Decl(typeParameterConstModifiersReturnsAndYields.ts, 7, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))

const result5 = test1(() => 101n);
>result5 : Symbol(result5, Decl(typeParameterConstModifiersReturnsAndYields.ts, 8, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))

const result6 = test1(() => false);
>result6 : Symbol(result6, Decl(typeParameterConstModifiersReturnsAndYields.ts, 9, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))

const result7 = test1(() => 11111);
>result7 : Symbol(result7, Decl(typeParameterConstModifiersReturnsAndYields.ts, 10, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))

const result8 = test1(() => E.Val);
>result8 : Symbol(result8, Decl(typeParameterConstModifiersReturnsAndYields.ts, 11, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))
>E.Val : Symbol(E.Val, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 8))
>E : Symbol(E, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 0))
>Val : Symbol(E.Val, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 8))

const result9 = test1(() => { return ['a']; });
>result9 : Symbol(result9, Decl(typeParameterConstModifiersReturnsAndYields.ts, 13, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))

const result10 = test1(() => { return `a${Math.random()}`; });
>result10 : Symbol(result10, Decl(typeParameterConstModifiersReturnsAndYields.ts, 14, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))
>Math.random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
>Math : Symbol(Math, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
>random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))

const result11 = test1(() => { return 'a'; });
>result11 : Symbol(result11, Decl(typeParameterConstModifiersReturnsAndYields.ts, 15, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))

const result12 = test1(() => { return true; });
>result12 : Symbol(result12, Decl(typeParameterConstModifiersReturnsAndYields.ts, 16, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))

const result13 = test1(() => { return 101n; });
>result13 : Symbol(result13, Decl(typeParameterConstModifiersReturnsAndYields.ts, 17, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))

const result14 = test1(() => { return false; });
>result14 : Symbol(result14, Decl(typeParameterConstModifiersReturnsAndYields.ts, 18, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))

const result15 = test1(() => { return 11111; });
>result15 : Symbol(result15, Decl(typeParameterConstModifiersReturnsAndYields.ts, 19, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))

const result16 = test1(() => { return E.Val; });
>result16 : Symbol(result16, Decl(typeParameterConstModifiersReturnsAndYields.ts, 20, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))
>E.Val : Symbol(E.Val, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 8))
>E : Symbol(E, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 0))
>Val : Symbol(E.Val, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 8))

const result17 = test1(async () => 'foo');
>result17 : Symbol(result17, Decl(typeParameterConstModifiersReturnsAndYields.ts, 22, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))

const result18 = test1(async () => { return 'foo'; });
>result18 : Symbol(result18, Decl(typeParameterConstModifiersReturnsAndYields.ts, 23, 5))
>test1 : Symbol(test1, Decl(typeParameterConstModifiersReturnsAndYields.ts, 0, 20))

declare function test2<const T>(create: () => Promise<T>): T;
>test2 : Symbol(test2, Decl(typeParameterConstModifiersReturnsAndYields.ts, 23, 54))
>T : Symbol(T, Decl(typeParameterConstModifiersReturnsAndYields.ts, 25, 23))
>create : Symbol(create, Decl(typeParameterConstModifiersReturnsAndYields.ts, 25, 32))
>Promise : Symbol(Promise, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.promise.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2018.promise.d.ts, --, --))
>T : Symbol(T, Decl(typeParameterConstModifiersReturnsAndYields.ts, 25, 23))
>T : Symbol(T, Decl(typeParameterConstModifiersReturnsAndYields.ts, 25, 23))

const result19 = test2(async () => 'foo');
>result19 : Symbol(result19, Decl(typeParameterConstModifiersReturnsAndYields.ts, 27, 5))
>test2 : Symbol(test2, Decl(typeParameterConstModifiersReturnsAndYields.ts, 23, 54))

const result20 = test2(async () => { return 'foo'; });
>result20 : Symbol(result20, Decl(typeParameterConstModifiersReturnsAndYields.ts, 28, 5))
>test2 : Symbol(test2, Decl(typeParameterConstModifiersReturnsAndYields.ts, 23, 54))

declare function test3<const T, const R>(arg: () => Generator<T, R>): [T, R]
>test3 : Symbol(test3, Decl(typeParameterConstModifiersReturnsAndYields.ts, 28, 54))
>T : Symbol(T, Decl(typeParameterConstModifiersReturnsAndYields.ts, 30, 23))
>R : Symbol(R, Decl(typeParameterConstModifiersReturnsAndYields.ts, 30, 31))
>arg : Symbol(arg, Decl(typeParameterConstModifiersReturnsAndYields.ts, 30, 41))
>Generator : Symbol(Generator, Decl(lib.es2015.generator.d.ts, --, --))
>T : Symbol(T, Decl(typeParameterConstModifiersReturnsAndYields.ts, 30, 23))
>R : Symbol(R, Decl(typeParameterConstModifiersReturnsAndYields.ts, 30, 31))
>T : Symbol(T, Decl(typeParameterConstModifiersReturnsAndYields.ts, 30, 23))
>R : Symbol(R, Decl(typeParameterConstModifiersReturnsAndYields.ts, 30, 31))

const result21 = test3(function*() {
>result21 : Symbol(result21, Decl(typeParameterConstModifiersReturnsAndYields.ts, 32, 5))
>test3 : Symbol(test3, Decl(typeParameterConstModifiersReturnsAndYields.ts, 28, 54))

yield 10;
return '1';
});

declare function test4<const T, const R>(arg: () => AsyncGenerator<T, R>): [T, R]
>test4 : Symbol(test4, Decl(typeParameterConstModifiersReturnsAndYields.ts, 35, 3))
>T : Symbol(T, Decl(typeParameterConstModifiersReturnsAndYields.ts, 37, 23))
>R : Symbol(R, Decl(typeParameterConstModifiersReturnsAndYields.ts, 37, 31))
>arg : Symbol(arg, Decl(typeParameterConstModifiersReturnsAndYields.ts, 37, 41))
>AsyncGenerator : Symbol(AsyncGenerator, Decl(lib.es2018.asyncgenerator.d.ts, --, --))
>T : Symbol(T, Decl(typeParameterConstModifiersReturnsAndYields.ts, 37, 23))
>R : Symbol(R, Decl(typeParameterConstModifiersReturnsAndYields.ts, 37, 31))
>T : Symbol(T, Decl(typeParameterConstModifiersReturnsAndYields.ts, 37, 23))
>R : Symbol(R, Decl(typeParameterConstModifiersReturnsAndYields.ts, 37, 31))

const result22 = test4(async function*() {
>result22 : Symbol(result22, Decl(typeParameterConstModifiersReturnsAndYields.ts, 39, 5))
>test4 : Symbol(test4, Decl(typeParameterConstModifiersReturnsAndYields.ts, 35, 3))

yield 10;
return '1';
});

// https://github.com/microsoft/TypeScript/issues/53813
const UploadThingServerHelper = <const ValidRoutes,>(route: {
>UploadThingServerHelper : Symbol(UploadThingServerHelper, Decl(typeParameterConstModifiersReturnsAndYields.ts, 45, 5))
>ValidRoutes : Symbol(ValidRoutes, Decl(typeParameterConstModifiersReturnsAndYields.ts, 45, 33))
>route : Symbol(route, Decl(typeParameterConstModifiersReturnsAndYields.ts, 45, 53))

readonly [Route in keyof ValidRoutes]: {
>Route : Symbol(Route, Decl(typeParameterConstModifiersReturnsAndYields.ts, 46, 12))
>ValidRoutes : Symbol(ValidRoutes, Decl(typeParameterConstModifiersReturnsAndYields.ts, 45, 33))

middleware: () => ValidRoutes[Route];
>middleware : Symbol(middleware, Decl(typeParameterConstModifiersReturnsAndYields.ts, 46, 42))
>ValidRoutes : Symbol(ValidRoutes, Decl(typeParameterConstModifiersReturnsAndYields.ts, 45, 33))
>Route : Symbol(Route, Decl(typeParameterConstModifiersReturnsAndYields.ts, 46, 12))

onUpload: (response: { metadata: ValidRoutes[Route] }) => void;
>onUpload : Symbol(onUpload, Decl(typeParameterConstModifiersReturnsAndYields.ts, 47, 41))
>response : Symbol(response, Decl(typeParameterConstModifiersReturnsAndYields.ts, 48, 15))
>metadata : Symbol(metadata, Decl(typeParameterConstModifiersReturnsAndYields.ts, 48, 26))
>ValidRoutes : Symbol(ValidRoutes, Decl(typeParameterConstModifiersReturnsAndYields.ts, 45, 33))
>Route : Symbol(Route, Decl(typeParameterConstModifiersReturnsAndYields.ts, 46, 12))

};
}) => {};

const FileRouter = UploadThingServerHelper({
>FileRouter : Symbol(FileRouter, Decl(typeParameterConstModifiersReturnsAndYields.ts, 52, 5))
>UploadThingServerHelper : Symbol(UploadThingServerHelper, Decl(typeParameterConstModifiersReturnsAndYields.ts, 45, 5))

example: {
>example : Symbol(example, Decl(typeParameterConstModifiersReturnsAndYields.ts, 52, 44))

middleware: () => "someValue",
>middleware : Symbol(middleware, Decl(typeParameterConstModifiersReturnsAndYields.ts, 53, 12))

onUpload: (response) => {
>onUpload : Symbol(onUpload, Decl(typeParameterConstModifiersReturnsAndYields.ts, 54, 34))
>response : Symbol(response, Decl(typeParameterConstModifiersReturnsAndYields.ts, 55, 15))

const v: "someValue" = response.metadata;
>v : Symbol(v, Decl(typeParameterConstModifiersReturnsAndYields.ts, 56, 11))
>response.metadata : Symbol(metadata, Decl(typeParameterConstModifiersReturnsAndYields.ts, 48, 26))
>response : Symbol(response, Decl(typeParameterConstModifiersReturnsAndYields.ts, 55, 15))
>metadata : Symbol(metadata, Decl(typeParameterConstModifiersReturnsAndYields.ts, 48, 26))

},
},
});

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