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Revert return type narrowing #61136

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Feb 7, 2025
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Revert return type narrowing #61136

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377 changes: 19 additions & 358 deletions src/compiler/checker.ts
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11 changes: 2 additions & 9 deletions src/compiler/types.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -6249,7 +6249,6 @@ export interface NodeLinks {
decoratorSignature?: Signature; // Signature for decorator as if invoked by the runtime.
spreadIndices?: { first: number | undefined, last: number | undefined }; // Indices of first and last spread elements in array literal
parameterInitializerContainsUndefined?: boolean; // True if this is a parameter declaration whose type annotation contains "undefined".
contextualReturnType?: Type; // If the node is a return statement's expression, then this is the contextual return type.
fakeScopeForSignatureDeclaration?: "params" | "typeParams"; // If present, this is a fake scope injected into an enclosing declaration chain.
assertionExpressionType?: Type; // Cached type of the expression of a type assertion
potentialThisCollisions?: Node[];
Expand Down Expand Up @@ -6517,8 +6516,6 @@ export const enum ObjectFlags {
IsGenericIndexType = 1 << 23, // Union or intersection contains generic index type
/** @internal */
IsGenericType = IsGenericObjectType | IsGenericIndexType,
/** @internal */
IsNarrowingType = 1 << 24, // Substitution type that comes from type narrowing

// Flags that require TypeFlags.Union
/** @internal */
Expand Down Expand Up @@ -6918,16 +6915,12 @@ export interface StringMappingType extends InstantiableType {
}

// Type parameter substitution (TypeFlags.Substitution)
// - Substitution types are created for type parameters or indexed access types that occur in the
// Substitution types are created for type parameters or indexed access types that occur in the
// true branch of a conditional type. For example, in 'T extends string ? Foo<T> : Bar<T>', the
// reference to T in Foo<T> is resolved as a substitution type that substitutes 'string & T' for T.
// Thus, if Foo has a 'string' constraint on its type parameter, T will satisfy it.
// - Substitution types are also created for NoInfer<T> types. Those are represented as substitution
// Substitution type are also created for NoInfer<T> types. Those are represented as substitution
// types where the constraint is type 'unknown' (which is never generated for the case above).
// - Substitution types are also created for return type narrowing:
// if a type parameter `T` is linked to a parameter `x` and `x`'s narrowed type is `S`,
// we represent that with a substitution type with base `T` and constraint `S`.
// The resulting substitution type has `ObjectFlags.IsNarrowedType` set.
export interface SubstitutionType extends InstantiableType {
objectFlags: ObjectFlags;
baseType: Type; // Target type
Expand Down
36 changes: 36 additions & 0 deletions tests/baselines/reference/conditionalReturnExpression.errors.txt
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@@ -0,0 +1,36 @@
conditionalReturnExpression.ts(2,18): error TS2322: Type '1' is not assignable to type '3'.
conditionalReturnExpression.ts(2,23): error TS2322: Type '2' is not assignable to type '3'.
conditionalReturnExpression.ts(8,43): error TS2322: Type 'number' is not assignable to type 'string'.
conditionalReturnExpression.ts(19,71): error TS2322: Type 'number' is not assignable to type 'string'.


==== conditionalReturnExpression.ts (4 errors) ====
function return1(x: boolean): 3 {
return (x ? (1) : 2);
~
!!! error TS2322: Type '1' is not assignable to type '3'.
~
!!! error TS2322: Type '2' is not assignable to type '3'.
}

declare function getAny(): any;

function return2(x: string): string {
return x.startsWith("a") ? getAny() : 1;
~
!!! error TS2322: Type 'number' is not assignable to type 'string'.
}

function return3(x: string): string {
return x.startsWith("a") ? "a" : x;
}

function return4(x: string): string {
return (x.startsWith("a") ? getAny() : 1) as string;
}

const return5 = (x: string): string => x.startsWith("a") ? getAny() : 1;
~
!!! error TS2322: Type 'number' is not assignable to type 'string'.

const return6 = (x: string): string => (x.startsWith("a") ? getAny() : 1) as string;
63 changes: 63 additions & 0 deletions tests/baselines/reference/conditionalReturnExpression.symbols
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@@ -0,0 +1,63 @@
//// [tests/cases/compiler/conditionalReturnExpression.ts] ////

=== conditionalReturnExpression.ts ===
function return1(x: boolean): 3 {
>return1 : Symbol(return1, Decl(conditionalReturnExpression.ts, 0, 0))
>x : Symbol(x, Decl(conditionalReturnExpression.ts, 0, 17))

return (x ? (1) : 2);
>x : Symbol(x, Decl(conditionalReturnExpression.ts, 0, 17))
}

declare function getAny(): any;
>getAny : Symbol(getAny, Decl(conditionalReturnExpression.ts, 2, 1))

function return2(x: string): string {
>return2 : Symbol(return2, Decl(conditionalReturnExpression.ts, 4, 31))
>x : Symbol(x, Decl(conditionalReturnExpression.ts, 6, 17))

return x.startsWith("a") ? getAny() : 1;
>x.startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --))
>x : Symbol(x, Decl(conditionalReturnExpression.ts, 6, 17))
>startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --))
>getAny : Symbol(getAny, Decl(conditionalReturnExpression.ts, 2, 1))
}

function return3(x: string): string {
>return3 : Symbol(return3, Decl(conditionalReturnExpression.ts, 8, 1))
>x : Symbol(x, Decl(conditionalReturnExpression.ts, 10, 17))

return x.startsWith("a") ? "a" : x;
>x.startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --))
>x : Symbol(x, Decl(conditionalReturnExpression.ts, 10, 17))
>startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --))
>x : Symbol(x, Decl(conditionalReturnExpression.ts, 10, 17))
}

function return4(x: string): string {
>return4 : Symbol(return4, Decl(conditionalReturnExpression.ts, 12, 1))
>x : Symbol(x, Decl(conditionalReturnExpression.ts, 14, 17))

return (x.startsWith("a") ? getAny() : 1) as string;
>x.startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --))
>x : Symbol(x, Decl(conditionalReturnExpression.ts, 14, 17))
>startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --))
>getAny : Symbol(getAny, Decl(conditionalReturnExpression.ts, 2, 1))
}

const return5 = (x: string): string => x.startsWith("a") ? getAny() : 1;
>return5 : Symbol(return5, Decl(conditionalReturnExpression.ts, 18, 5))
>x : Symbol(x, Decl(conditionalReturnExpression.ts, 18, 17))
>x.startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --))
>x : Symbol(x, Decl(conditionalReturnExpression.ts, 18, 17))
>startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --))
>getAny : Symbol(getAny, Decl(conditionalReturnExpression.ts, 2, 1))

const return6 = (x: string): string => (x.startsWith("a") ? getAny() : 1) as string;
>return6 : Symbol(return6, Decl(conditionalReturnExpression.ts, 20, 5))
>x : Symbol(x, Decl(conditionalReturnExpression.ts, 20, 17))
>x.startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --))
>x : Symbol(x, Decl(conditionalReturnExpression.ts, 20, 17))
>startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --))
>getAny : Symbol(getAny, Decl(conditionalReturnExpression.ts, 2, 1))

167 changes: 167 additions & 0 deletions tests/baselines/reference/conditionalReturnExpression.types
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@@ -0,0 +1,167 @@
//// [tests/cases/compiler/conditionalReturnExpression.ts] ////

=== conditionalReturnExpression.ts ===
function return1(x: boolean): 3 {
>return1 : (x: boolean) => 3
> : ^ ^^ ^^^^^
>x : boolean
> : ^^^^^^^

return (x ? (1) : 2);
>(x ? (1) : 2) : 1 | 2
> : ^^^^^
>x ? (1) : 2 : 1 | 2
> : ^^^^^
>x : boolean
> : ^^^^^^^
>(1) : 1
> : ^
>1 : 1
> : ^
>2 : 2
> : ^
}

declare function getAny(): any;
>getAny : () => any
> : ^^^^^^

function return2(x: string): string {
>return2 : (x: string) => string
> : ^ ^^ ^^^^^
>x : string
> : ^^^^^^

return x.startsWith("a") ? getAny() : 1;
>x.startsWith("a") ? getAny() : 1 : any
> : ^^^
>x.startsWith("a") : boolean
> : ^^^^^^^
>x.startsWith : (searchString: string, position?: number) => boolean
> : ^ ^^ ^^ ^^^ ^^^^^
>x : string
> : ^^^^^^
>startsWith : (searchString: string, position?: number) => boolean
> : ^ ^^ ^^ ^^^ ^^^^^
>"a" : "a"
> : ^^^
>getAny() : any
> : ^^^
>getAny : () => any
> : ^^^^^^
>1 : 1
> : ^
}

function return3(x: string): string {
>return3 : (x: string) => string
> : ^ ^^ ^^^^^
>x : string
> : ^^^^^^

return x.startsWith("a") ? "a" : x;
>x.startsWith("a") ? "a" : x : string
> : ^^^^^^
>x.startsWith("a") : boolean
> : ^^^^^^^
>x.startsWith : (searchString: string, position?: number) => boolean
> : ^ ^^ ^^ ^^^ ^^^^^
>x : string
> : ^^^^^^
>startsWith : (searchString: string, position?: number) => boolean
> : ^ ^^ ^^ ^^^ ^^^^^
>"a" : "a"
> : ^^^
>"a" : "a"
> : ^^^
>x : string
> : ^^^^^^
}

function return4(x: string): string {
>return4 : (x: string) => string
> : ^ ^^ ^^^^^
>x : string
> : ^^^^^^

return (x.startsWith("a") ? getAny() : 1) as string;
>(x.startsWith("a") ? getAny() : 1) as string : string
> : ^^^^^^
>(x.startsWith("a") ? getAny() : 1) : any
> : ^^^
>x.startsWith("a") ? getAny() : 1 : any
> : ^^^
>x.startsWith("a") : boolean
> : ^^^^^^^
>x.startsWith : (searchString: string, position?: number) => boolean
> : ^ ^^ ^^ ^^^ ^^^^^
>x : string
> : ^^^^^^
>startsWith : (searchString: string, position?: number) => boolean
> : ^ ^^ ^^ ^^^ ^^^^^
>"a" : "a"
> : ^^^
>getAny() : any
> : ^^^
>getAny : () => any
> : ^^^^^^
>1 : 1
> : ^
}

const return5 = (x: string): string => x.startsWith("a") ? getAny() : 1;
>return5 : (x: string) => string
> : ^ ^^ ^^^^^
>(x: string): string => x.startsWith("a") ? getAny() : 1 : (x: string) => string
> : ^ ^^ ^^^^^
>x : string
> : ^^^^^^
>x.startsWith("a") ? getAny() : 1 : any
> : ^^^
>x.startsWith("a") : boolean
> : ^^^^^^^
>x.startsWith : (searchString: string, position?: number) => boolean
> : ^ ^^ ^^ ^^^ ^^^^^
>x : string
> : ^^^^^^
>startsWith : (searchString: string, position?: number) => boolean
> : ^ ^^ ^^ ^^^ ^^^^^
>"a" : "a"
> : ^^^
>getAny() : any
> : ^^^
>getAny : () => any
> : ^^^^^^
>1 : 1
> : ^

const return6 = (x: string): string => (x.startsWith("a") ? getAny() : 1) as string;
>return6 : (x: string) => string
> : ^ ^^ ^^^^^
>(x: string): string => (x.startsWith("a") ? getAny() : 1) as string : (x: string) => string
> : ^ ^^ ^^^^^
>x : string
> : ^^^^^^
>(x.startsWith("a") ? getAny() : 1) as string : string
> : ^^^^^^
>(x.startsWith("a") ? getAny() : 1) : any
> : ^^^
>x.startsWith("a") ? getAny() : 1 : any
> : ^^^
>x.startsWith("a") : boolean
> : ^^^^^^^
>x.startsWith : (searchString: string, position?: number) => boolean
> : ^ ^^ ^^ ^^^ ^^^^^
>x : string
> : ^^^^^^
>startsWith : (searchString: string, position?: number) => boolean
> : ^ ^^ ^^ ^^^ ^^^^^
>"a" : "a"
> : ^^^
>getAny() : any
> : ^^^
>getAny : () => any
> : ^^^^^^
>1 : 1
> : ^

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