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Merged
Jake Bailey (jakebailey) merged 3 commits into from
Oct 30, 2025
Merged

Allow implicit undefined returns when the contextual union type contains it #57912

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c309c6f
Allow implicit `undefined` returns when the contextual union type con…
Andarist Mar 22, 2024
4042168
Merge remote-tracking branch 'origin/main' into implicit-undefined-re…
Andarist May 1, 2024
7b63236
Merge remote-tracking branch 'origin/main' into implicit-undefined-re…
Andarist Oct 30, 2025
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2 changes: 1 addition & 1 deletion src/compiler/checker.ts
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Expand Up@@ -39125,7 +39125,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
if (types.length === 0) {
// For an async function, the return type will not be void/undefined, but rather a Promise for void/undefined.
const contextualReturnType = getContextualReturnType(func, /*contextFlags*/ undefined);
const returnType = contextualReturnType && (unwrapReturnType(contextualReturnType, functionFlags) || voidType).flags & TypeFlags.Undefined ? undefinedType : voidType;
const returnType = contextualReturnType && someType(unwrapReturnType(contextualReturnType, functionFlags) || voidType, t => !!(t.flags & TypeFlags.Undefined)) ? undefinedType : voidType;
return functionFlags & FunctionFlags.Async ? createPromiseReturnType(func, returnType) : // Async function
returnType; // Normal function
}
Expand Down
25 changes: 18 additions & 7 deletions ...lines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.errors.txt
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@@ -1,7 +1,5 @@
functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(5,17): error TS2355: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(9,17): error TS2355: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(17,7): error TS2322: Type '() => void' is not assignable to type '() => number | undefined'.
Type 'void' is not assignable to type 'number | undefined'.
functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(21,7): error TS2322: Type '() => void' is not assignable to type '() => number'.
Type 'void' is not assignable to type 'number'.
functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(29,23): error TS2355: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
Expand All@@ -10,7 +8,7 @@ functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(52,3): error T
Type 'void' is not assignable to type 'undefined'.


==== functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts (7 errors) ====
==== functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts (6 errors) ====
function f10(): undefined {
// Ok, return type allows implicit return of undefined
}
Expand All@@ -32,10 +30,7 @@ functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(52,3): error T
}

const f21: () => undefined | number = () => {
~~~
!!! error TS2322: Type '() => void' is not assignable to type '() => number | undefined'.
!!! error TS2322: Type 'void' is not assignable to type 'number | undefined'.
// Error, regular void function because contextual type for implicit return isn't just undefined
// Ok, contextual type for implicit return contains undefined
}
Comment on lines 32 to 34

@andrewbranchAndrew Branch (andrewbranch)Jun 18, 2024

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I’m not the right person to review this, because I personally lean strongly toward the belief that distinctions between return, return undefined, and no return belong in a linter and not in TS, which I think is not the consensus view on the team. I can’t tell whether the design meeting notes on this were meant to be descriptive or prescriptive:

But no implicit returns when you have 4 | undefined

If it was intended to be prescriptive, this baseline change violates the intention that was captured in the notes. However, moving in this direction is my preference, so I’m going to approve in hopes that it moves the review along 🤷‍♂️

@jakebaileyJake Bailey (jakebailey)Jun 18, 2024

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I believe that was descriptive, stating what the current behavior is and that it's inconsistent.


const f22: () => number = () => {
Expand DownExpand Up@@ -85,4 +80,20 @@ functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(52,3): error T
}

f(h2);

// https://github.com/microsoft/TypeScript/issues/57840

type FN = () => Promise<undefined> | undefined;

const fn1: FN = () => {
return;
};

const fn2: FN = async () => {
return;
};

const fn3: FN = () => {};

const fn4: FN = async () => {};

28 changes: 26 additions & 2 deletions tests/baselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.js
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Expand Up@@ -18,7 +18,7 @@ const f20: () => undefined = () => {
}

const f21: () => undefined | number = () => {
// Error, regular void function because contextual type for implicit return isn't just undefined
// Ok, contextual type for implicit return contains undefined
}

const f22: () => number = () => {
Expand DownExpand Up@@ -58,6 +58,22 @@ function h2(): undefined {
}

f(h2);

// https://github.com/microsoft/TypeScript/issues/57840

type FN = () => Promise<undefined> | undefined;

const fn1: FN = () => {
return;
};

const fn2: FN = async () => {
return;
};

const fn3: FN = () => {};

const fn4: FN = async () => {};


//// [functionsMissingReturnStatementsAndExpressionsStrictNullChecks.js]
Expand All@@ -74,7 +90,7 @@ const f20 = () => {
// Ok, contextual type for implicit return is undefined
};
const f21 = () => {
// Error, regular void function because contextual type for implicit return isn't just undefined
// Ok, contextual type for implicit return contains undefined
};
const f22 = () => {
// Error, regular void function because contextual type for implicit return isn't just undefined
Expand All@@ -98,3 +114,11 @@ f(h1); // Error
function h2() {
}
f(h2);
const fn1 = () => {
return;
};
const fn2 = async () => {
return;
};
const fn3 = () => { };
const fn4 = async () => { };
30 changes: 29 additions & 1 deletion ...aselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.symbols
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Expand Up@@ -28,7 +28,7 @@ const f20: () => undefined = () => {
const f21: () => undefined | number = () => {
>f21 : Symbol(f21, Decl(functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts, 16, 5))

// Error, regular void function because contextual type for implicit return isn't just undefined
// Ok, contextual type for implicit return contains undefined
}

const f22: () => number = () => {
Expand DownExpand Up@@ -92,3 +92,31 @@ f(h2);
>f : Symbol(f, Decl(functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts, 34, 1))
>h2 : Symbol(h2, Decl(functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts, 51, 6))

// https://github.com/microsoft/TypeScript/issues/57840

type FN = () => Promise<undefined> | undefined;
>FN : Symbol(FN, Decl(functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts, 56, 6))
>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, --, --))

const fn1: FN = () => {
>fn1 : Symbol(fn1, Decl(functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts, 62, 5))
>FN : Symbol(FN, Decl(functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts, 56, 6))

return;
};

const fn2: FN = async () => {
>fn2 : Symbol(fn2, Decl(functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts, 66, 5))
>FN : Symbol(FN, Decl(functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts, 56, 6))

return;
};

const fn3: FN = () => {};
>fn3 : Symbol(fn3, Decl(functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts, 70, 5))
>FN : Symbol(FN, Decl(functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts, 56, 6))

const fn4: FN = async () => {};
>fn4 : Symbol(fn4, Decl(functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts, 72, 5))
>FN : Symbol(FN, Decl(functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts, 56, 6))

42 changes: 39 additions & 3 deletions .../baselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.types
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Expand Up@@ -34,10 +34,10 @@ const f20: () => undefined = () => {
const f21: () => undefined | number = () => {
>f21 : () => undefined | number
> : ^^^^^^
>() => { // Error, regular void function because contextual type for implicit return isn't just undefined} : () => void
> : ^^^^^^^^^^
>() => { // Ok, contextual type for implicit return contains undefined} : () => undefined
> : ^^^^^^^^^^^^^^^

// Error, regular void function because contextual type for implicit return isn't just undefined
// Ok, contextual type for implicit return contains undefined
}

const f22: () => number = () => {
Expand DownExpand Up@@ -132,3 +132,39 @@ f(h2);
>h2 : () => undefined
> : ^^^^^^

// https://github.com/microsoft/TypeScript/issues/57840

type FN = () => Promise<undefined> | undefined;
>FN : FN
> : ^^

const fn1: FN = () => {
>fn1 : FN
> : ^^
>() => { return;} : () => undefined
> : ^^^^^^^^^^^^^^^

return;
};

const fn2: FN = async () => {
>fn2 : FN
> : ^^
>async () => { return;} : () => Promise<undefined>
> : ^^^^^^^^^^^^^^^^^^^^^^^^

return;
};

const fn3: FN = () => {};
>fn3 : FN
> : ^^
>() => {} : () => undefined
> : ^^^^^^^^^^^^^^^

const fn4: FN = async () => {};
>fn4 : FN
> : ^^
>async () => {} : () => Promise<undefined>
> : ^^^^^^^^^^^^^^^^^^^^^^^^

16 changes: 8 additions & 8 deletions tests/baselines/reference/inferenceDoesNotAddUndefinedOrNull.types
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Expand Up@@ -46,16 +46,16 @@ function flatMapChildren<T>(node: Node, cb: (child: Node) => readonly T[] | T |
> : ^^^^^^^

node.forEachChild(child => {
>node.forEachChild(child => { const value = cb(child); if (value !== undefined) { result.push(...toArray(value)); } }) : void | undefined
> : ^^^^^^^^^^^^^^^^
>node.forEachChild(child => { const value = cb(child); if (value !== undefined) { result.push(...toArray(value)); } }) : undefined
> : ^^^^^^^^^
>node.forEachChild : <T_1>(cbNode: (node: Node) => T_1 | undefined, cbNodeArray?: (nodes: NodeArray<Node>) => T_1 | undefined) => T_1 | undefined
> : ^^^^^^ ^^ ^^ ^^^ ^^^^^
>node : Node
> : ^^^^
>forEachChild : <T_1>(cbNode: (node: Node) => T_1 | undefined, cbNodeArray?: (nodes: NodeArray<Node>) => T_1 | undefined) => T_1 | undefined
> : ^^^^^^ ^^ ^^ ^^^ ^^^^^
>child => { const value = cb(child); if (value !== undefined) { result.push(...toArray(value)); } } : (child: Node) => void
> : ^ ^^^^^^^^^^^^^^^
>child => { const value = cb(child); if (value !== undefined) { result.push(...toArray(value)); } } : (child: Node) => undefined
> : ^ ^^^^^^^^^^^^^^^^^^^^
>child : Node
> : ^^^^

Expand DownExpand Up@@ -118,16 +118,16 @@ function flatMapChildren2<T>(node: Node, cb: (child: Node) => readonly T[] | T |
> : ^^^^^^^

node.forEachChild(child => {
>node.forEachChild(child => { const value = cb(child); if (value !== null) { result.push(...toArray(value)); } }) : void | undefined
> : ^^^^^^^^^^^^^^^^
>node.forEachChild(child => { const value = cb(child); if (value !== null) { result.push(...toArray(value)); } }) : undefined
> : ^^^^^^^^^
>node.forEachChild : <T_1>(cbNode: (node: Node) => T_1 | undefined, cbNodeArray?: (nodes: NodeArray<Node>) => T_1 | undefined) => T_1 | undefined
> : ^^^^^^ ^^ ^^ ^^^ ^^^^^
>node : Node
> : ^^^^
>forEachChild : <T_1>(cbNode: (node: Node) => T_1 | undefined, cbNodeArray?: (nodes: NodeArray<Node>) => T_1 | undefined) => T_1 | undefined
> : ^^^^^^ ^^ ^^ ^^^ ^^^^^
>child => { const value = cb(child); if (value !== null) { result.push(...toArray(value)); } } : (child: Node) => void
> : ^ ^^^^^^^^^^^^^^^
>child => { const value = cb(child); if (value !== null) { result.push(...toArray(value)); } } : (child: Node) => undefined
> : ^ ^^^^^^^^^^^^^^^^^^^^
>child : Node
> : ^^^^

Expand Down
18 changes: 17 additions & 1 deletion tests/cases/compiler/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts
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Expand Up@@ -19,7 +19,7 @@ const f20: () => undefined = () => {
}

const f21: () => undefined | number = () => {
// Error, regular void function because contextual type for implicit return isn't just undefined
// Ok, contextual type for implicit return contains undefined
}

const f22: () => number = () => {
Expand DownExpand Up@@ -59,3 +59,19 @@ function h2(): undefined {
}

f(h2);

// https://github.com/microsoft/TypeScript/issues/57840

type FN = () => Promise<undefined> | undefined;

const fn1: FN = () => {
return;
};

const fn2: FN = async () => {
return;
};

const fn3: FN = () => {};

const fn4: FN = async () => {};