Allow implicit undefined returns when the contextual union type contains it

Port of TypeScript PR #57912. Changed the checker to use someType() instead
of a direct flag check when determining if an implicit return should produce
undefined vs void. This allows functions with a contextual return type that
is a union containing undefined to have implicit undefined returns.

Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Author: Copilot

internal/checker/checker.go

@@ -19700,7 +19700,7 @@ func (c *Checker) getReturnTypeFromBody(fn *ast.Node, checkMode CheckMode) *Type
 			// For an async function, the return type will not be void/undefined, but rather a Promise for void/undefined.
 			contextualReturnType := c.getContextualReturnType(fn, ContextFlagsNone)
 			var returnType *Type
-			if contextualReturnType != nil && core.OrElse(c.unwrapReturnType(contextualReturnType, functionFlags), c.voidType).flags&TypeFlagsUndefined != 0 {
+			if contextualReturnType != nil && someType(core.OrElse(c.unwrapReturnType(contextualReturnType, functionFlags), c.voidType), func(t *Type) bool { return t.flags&TypeFlagsUndefined != 0 }) {
 				returnType = c.undefinedType
 			} else {
 				returnType = c.voidType

testdata/baselines/reference/submodule/compiler/functionsMissingReturnStatementsAndExpressions(target=es2015).errors.txt

@@ -5,16 +5,12 @@ functionsMissingReturnStatementsAndExpressions.ts(45,5): error TS2322: Type 'nul
 functionsMissingReturnStatementsAndExpressions.ts(74,5): error TS2322: Type 'undefined' is not assignable to type 'number'.
 functionsMissingReturnStatementsAndExpressions.ts(99,17): error TS2355: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
 functionsMissingReturnStatementsAndExpressions.ts(107,17): error TS2355: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
-functionsMissingReturnStatementsAndExpressions.ts(120,7): error TS2322: Type '() => void' is not assignable to type '() => number | undefined'.
-  Type 'void' is not assignable to type 'number | undefined'.
-functionsMissingReturnStatementsAndExpressions.ts(124,7): error TS2322: Type '() => void' is not assignable to type '() => number | undefined'.
-  Type 'void' is not assignable to type 'number | undefined'.
 functionsMissingReturnStatementsAndExpressions.ts(130,16): error TS2378: A 'get' accessor must return a value.
 functionsMissingReturnStatementsAndExpressions.ts(152,15): error TS18050: The value 'undefined' cannot be used here.
 functionsMissingReturnStatementsAndExpressions.ts(153,5): error TS1003: Identifier expected.
 
 
-==== functionsMissingReturnStatementsAndExpressions.ts (12 errors) ====
+==== functionsMissingReturnStatementsAndExpressions.ts (10 errors) ====
     function f1(): string {
                    ~~~~~~
 !!! error TS2355: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
@@ -149,16 +145,10 @@ functionsMissingReturnStatementsAndExpressions.ts(153,5): error TS1003: Identifi
     }
 
     const f32: () => 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, contextual type for implicit return isn't just `undefined`
     }
 
     const f33: () => 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, contextual type for expression-less return isn't just `undefined`
         return;
     }

testdata/baselines/reference/submodule/compiler/functionsMissingReturnStatementsAndExpressions(target=es2015).errors.txt.diff

@@ -185,14 +185,14 @@ const f31: () => undefined = () => {
 
 const f32: () => undefined | number = () => {
 >f32 : () => number | undefined
->() => {    // Error, contextual type for implicit return isn't just `undefined`} : () => void
+>() => {    // Error, contextual type for implicit return isn't just `undefined`} : () => undefined
 
     // Error, contextual type for implicit return isn't just `undefined`
 }
 
 const f33: () => undefined | number = () => {
 >f33 : () => number | undefined
->() => {    // Error, contextual type for expression-less return isn't just `undefined`    return;} : () => void
+>() => {    // Error, contextual type for expression-less return isn't just `undefined`    return;} : () => undefined
 
     // Error, contextual type for expression-less return isn't just `undefined`
     return;

testdata/baselines/reference/submodule/compiler/functionsMissingReturnStatementsAndExpressions(target=es2015).types

@@ -30,18 +30,15 @@
 
  const f32: () => undefined | number = () => {
 ->f32 : () => undefined | number
-->() => {    // Error, contextual type for implicit return isn't just `undefined`} : () => undefined
 +>f32 : () => number | undefined
-+>() => {    // Error, contextual type for implicit return isn't just `undefined`} : () => void
+ >() => {    // Error, contextual type for implicit return isn't just `undefined`} : () => undefined
 
      // Error, contextual type for implicit return isn't just `undefined`
  }
 
  const f33: () => undefined | number = () => {
 ->f33 : () => undefined | number
-->() => {    // Error, contextual type for expression-less return isn't just `undefined`    return;} : () => undefined
 +>f33 : () => number | undefined
-+>() => {    // Error, contextual type for expression-less return isn't just `undefined`    return;} : () => void
+ >() => {    // Error, contextual type for expression-less return isn't just `undefined`    return;} : () => undefined
 
-     // Error, contextual type for expression-less return isn't just `undefined`
-     return;
+     // Error, contextual type for expression-less return isn't just `undefined`

testdata/baselines/reference/submodule/compiler/functionsMissingReturnStatementsAndExpressions(target=es2015).types.diff

@@ -1,20 +1,14 @@
 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.
 functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(33,23): error TS2355: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
 functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(52,3): error TS2345: Argument of type '() => void' is not assignable to parameter of type '() => undefined'.
   Type 'void' is not assignable to type 'undefined'.
-functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(63,7): error TS2322: Type '() => void' is not assignable to type 'FN'.
-  Type 'void' is not assignable to type 'Promise<undefined> | undefined'.
-functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(71,7): error TS2322: Type '() => void' is not assignable to type 'FN'.
-  Type 'void' is not assignable to type 'Promise<undefined> | undefined'.
 
 
-==== functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts (9 errors) ====
+==== functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts (6 errors) ====
     function f10(): undefined {
         // Ok, return type allows implicit return of undefined
     }
@@ -36,9 +30,6 @@ functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(71,7): 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'.
         // Ok, contextual type for implicit return contains undefined
     }
 
@@ -95,9 +86,6 @@ functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(71,7): error T
     type FN = () => Promise<undefined> | undefined;
 
     const fn1: FN = () => {
-          ~~~
-!!! error TS2322: Type '() => void' is not assignable to type 'FN'.
-!!! error TS2322:   Type 'void' is not assignable to type 'Promise<undefined> | undefined'.
         return;
     };
 
@@ -106,9 +94,6 @@ functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(71,7): error T
     };
 
     const fn3: FN = () => {};
-          ~~~
-!!! error TS2322: Type '() => void' is not assignable to type 'FN'.
-!!! error TS2322:   Type 'void' is not assignable to type 'Promise<undefined> | undefined'.
 
     const fn4: FN = async () => {};
 

testdata/baselines/reference/submodule/compiler/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.errors.txt

@@ -28,7 +28,7 @@ const f20: () => undefined = () => {
 
 const f21: () => undefined | number = () => {
 >f21 : () => number | undefined
->() => {    // Ok, contextual type for implicit return contains undefined} : () => void
+>() => {    // Ok, contextual type for implicit return contains undefined} : () => undefined
 
     // Ok, contextual type for implicit return contains undefined
 }
@@ -107,7 +107,7 @@ type FN = () => Promise<undefined> | undefined;
 
 const fn1: FN = () => {
 >fn1 : FN
->() => {    return;} : () => void
+>() => {    return;} : () => undefined
 
     return;
 };
@@ -121,7 +121,7 @@ const fn2: FN = async () => {
 
 const fn3: FN = () => {};
 >fn3 : FN
->() => {} : () => void
+>() => {} : () => undefined
 
 const fn4: FN = async () => {};
 >fn4 : FN

testdata/baselines/reference/submodule/compiler/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.errors.txt.diff

@@ -14,12 +14,10 @@
 
  const f21: () => undefined | number = () => {
 ->f21 : () => undefined | number
-->() => {    // Ok, contextual type for implicit return contains undefined} : () => undefined
 +>f21 : () => number | undefined
-+>() => {    // Ok, contextual type for implicit return contains undefined} : () => void
+ >() => {    // Ok, contextual type for implicit return contains undefined} : () => undefined
 
      // Ok, contextual type for implicit return contains undefined
- }
 @@= skipped -20, +20 lines =@@
  }
 
@@ -28,22 +26,4 @@
 +>f31 : () => Promise<number | undefined>
 
      // Error, return type isn't just undefined
- }
-@@= skipped -60, +60 lines =@@
-
- const fn1: FN = () => {
- >fn1 : FN
-->() => {    return;} : () => undefined
-+>() => {    return;} : () => void
-
-     return;
- };
-@@= skipped -14, +14 lines =@@
-
- const fn3: FN = () => {};
- >fn3 : FN
-->() => {} : () => undefined
-+>() => {} : () => void
-
- const fn4: FN = async () => {};
- >fn4 : FN
+ }

testdata/baselines/reference/submodule/compiler/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.types

@@ -31,11 +31,11 @@ function flatMapChildren<T>(node: Node, cb: (child: Node) => readonly T[] | T |
 >[] : never[]
 
     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) | undefined) => T_1 | undefined
 >node : Node
 >forEachChild : <T_1>(cbNode: (node: Node) => T_1 | undefined, cbNodeArray?: ((nodes: NodeArray<Node>) => T_1 | undefined) | 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
 
         const value = cb(child);
@@ -75,11 +75,11 @@ function flatMapChildren2<T>(node: Node, cb: (child: Node) => readonly T[] | T |
 >[] : never[]
 
     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) | undefined) => T_1 | undefined
 >node : Node
 >forEachChild : <T_1>(cbNode: (node: Node) => T_1 | undefined, cbNodeArray?: ((nodes: NodeArray<Node>) => T_1 | undefined) | 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
 
         const value = cb(child);