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Avoid rewriting homomorphic mapped types with homomorphic instantiations #53215

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Merged
weswigham merged 6 commits into from
Mar 21, 2023
Merged

Avoid rewriting homomorphic mapped types with homomorphic instantiations #53215

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a927933
Avoid rewriting mapped types that are already inline
Andarist Mar 12, 2023
f562093
tweak isHomomorphicMappedTypeWithNonHomomorphicInstantiation
Andarist Mar 15, 2023
5d51cf7
Avoid rewriting mapped types without a target
Andarist Mar 19, 2023
c9c4682
Merge remote-tracking branch 'origin/main' into fix/inline-mapped-typ…
Andarist Mar 19, 2023
587d9e7
implemented suggested changes
Andarist Mar 20, 2023
4b6afba
Merge remote-tracking branch 'origin/main' into fix/inline-mapped-typ…
Andarist Mar 20, 2023
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3 changes: 2 additions & 1 deletion src/compiler/checker.ts
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Expand Up @@ -6616,7 +6616,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {

function isHomomorphicMappedTypeWithNonHomomorphicInstantiation(type: MappedType) {
return isMappedTypeWithKeyofConstraintDeclaration(type)
&& !(getModifiersTypeFromMappedType(type).flags & TypeFlags.TypeParameter);
&& !(getModifiersTypeFromMappedType(type).flags & TypeFlags.TypeParameter)
&& !!type.aliasSymbol;
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@weswigham weswigham Mar 14, 2023

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This shouldn't have anything to do with the alias symbol as by the time we're trying to print with this function, the alias symbol has already been discarded as an option for printing the mapped type. Instead, what we care about is if the type will be picked up as a homomorphic type variable (or not). Type parameters always are, so were a conservative initial choice - it's generally safe to wrap anything else which is still recognized as a homomorphic type variable up and present it as a type parameter, too - all you get is unnecessarily verbose printback sometimes. Instead of checking the alias symbol here (which seems pretty unrelated), you want to check, rather than if the modifiers type is still a type parameter, if an index on the modifiers type still produces a homomorphic type variable. In this case, an index of a mapped type just returns the inner mapped type's constraint type, which can then be unwrapped, so it is recognized as homomorphic. So, instead of the simple, conservative !(getModifiersTypeFromMappedType(type).flags & TypeFlags.TypeParameter) check, you want something to more closely mirror getHomomorphicTypeVariable's conditions like

                let index: Type;
                return isMappedTypeWithKeyofConstraintDeclaration(type)
                    && (index = getIndexType(getModifiersTypeFromMappedType(type)), !(index.flags & TypeFlags.Index && (index as IndexType).type.flags & TypeFlags.TypeParameter)))

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@Andarist Andarist Mar 15, 2023

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I applied the suggested change but it reverted the change to the emit of this thing (TS playground):

export declare type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>;
export declare type PartialProperties<T, K extends keyof T> = Partial<Pick<T, K>> & Omit<T, K>;
export function doSomething_Actual<T extends {
    prop: string;
}>(a: T) {
    const x: { [P in keyof PartialProperties<T, "prop">]: PartialProperties<T, "prop">[P]; } = null as any;
    return x;
}

Does this one have to be rewritten? 🤔

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@weswigham weswigham Mar 15, 2023

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Probably not, since, in that case, I don't think PartialProperties is homomorphic, so the wrapping mapped type shouldn't be either. Right now we're using isMappedTypeWithKeyofConstraintDeclaration as a proxy for was-this-declared-as-homomorphic - we probably need to swap that out for something a bit more accurate, too. Potentially the same check we're doing on type but on type.target (and negated)?

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@weswigham weswigham Mar 15, 2023

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Indeed, it's not actually homomorphic -

doSomething_Actual<{prop: string, propb: string} | {prop: string, propa: string}>(null as any);

ends up with type

{
    prop?: string | undefined;
}

so we just need to tighten up that side of the check, too.

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@Andarist Andarist Mar 19, 2023

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This particular mapped type doesn't have a .target property - I might have misunderstood your recommendation for the proposed changes but for now, I've chosen to just avoid rewriting in case of !type.target. I think this is correct (it yields the expected result for this case and the rest of the test suite) but maybe I'm missing some further details.

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@weswigham weswigham Mar 20, 2023

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Actually, that's perfect - if the mapped type doesn't have a .target, then leaving it as-is is always correct - it's still it's declared form, uninstantiated.

}

function createMappedTypeNodeFromType(type: MappedType) {
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19 changes: 19 additions & 0 deletions tests/baselines/reference/mappedTypeGenericInstantiationPreservesInlineForm.js
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@@ -0,0 +1,19 @@
//// [mappedTypeGenericInstantiationPreservesInlineForm.ts]
// repro from #53109

export const test1 = <T = Record<string, never>>(schema: {
[K in keyof Required<T>]: T[K];
}) => {}

export function test2<T = Record<string, never>>(schema: {
[K in keyof Required<T>]: T[K];
}) {};




//// [mappedTypeGenericInstantiationPreservesInlineForm.d.ts]
export declare const test1: <T = Record<string, never>>(schema: { [K in keyof Required<T>]: T[K]; }) => void;
export declare function test2<T = Record<string, never>>(schema: {
[K in keyof Required<T>]: T[K];
}): void;
33 changes: 33 additions & 0 deletions tests/baselines/reference/mappedTypeGenericInstantiationPreservesInlineForm.symbols
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@@ -0,0 +1,33 @@
=== tests/cases/compiler/mappedTypeGenericInstantiationPreservesInlineForm.ts ===
// repro from #53109

export const test1 = <T = Record<string, never>>(schema: {
>test1 : Symbol(test1, Decl(mappedTypeGenericInstantiationPreservesInlineForm.ts, 2, 12))
>T : Symbol(T, Decl(mappedTypeGenericInstantiationPreservesInlineForm.ts, 2, 22))
>Record : Symbol(Record, Decl(lib.es5.d.ts, --, --))
>schema : Symbol(schema, Decl(mappedTypeGenericInstantiationPreservesInlineForm.ts, 2, 49))

[K in keyof Required<T>]: T[K];
>K : Symbol(K, Decl(mappedTypeGenericInstantiationPreservesInlineForm.ts, 3, 5))
>Required : Symbol(Required, Decl(lib.es5.d.ts, --, --))
>T : Symbol(T, Decl(mappedTypeGenericInstantiationPreservesInlineForm.ts, 2, 22))
>T : Symbol(T, Decl(mappedTypeGenericInstantiationPreservesInlineForm.ts, 2, 22))
>K : Symbol(K, Decl(mappedTypeGenericInstantiationPreservesInlineForm.ts, 3, 5))

}) => {}

export function test2<T = Record<string, never>>(schema: {
>test2 : Symbol(test2, Decl(mappedTypeGenericInstantiationPreservesInlineForm.ts, 4, 8))
>T : Symbol(T, Decl(mappedTypeGenericInstantiationPreservesInlineForm.ts, 6, 22))
>Record : Symbol(Record, Decl(lib.es5.d.ts, --, --))
>schema : Symbol(schema, Decl(mappedTypeGenericInstantiationPreservesInlineForm.ts, 6, 49))

[K in keyof Required<T>]: T[K];
>K : Symbol(K, Decl(mappedTypeGenericInstantiationPreservesInlineForm.ts, 7, 5))
>Required : Symbol(Required, Decl(lib.es5.d.ts, --, --))
>T : Symbol(T, Decl(mappedTypeGenericInstantiationPreservesInlineForm.ts, 6, 22))
>T : Symbol(T, Decl(mappedTypeGenericInstantiationPreservesInlineForm.ts, 6, 22))
>K : Symbol(K, Decl(mappedTypeGenericInstantiationPreservesInlineForm.ts, 7, 5))

}) {};

18 changes: 18 additions & 0 deletions tests/baselines/reference/mappedTypeGenericInstantiationPreservesInlineForm.types
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=== tests/cases/compiler/mappedTypeGenericInstantiationPreservesInlineForm.ts ===
// repro from #53109

export const test1 = <T = Record<string, never>>(schema: {
>test1 : <T = Record<string, never>>(schema: { [K in keyof Required<T>]: T[K]; }) => void
><T = Record<string, never>>(schema: { [K in keyof Required<T>]: T[K];}) => {} : <T = Record<string, never>>(schema: { [K in keyof Required<T>]: T[K]; }) => void
>schema : { [K in keyof Required<T>]: T[K]; }

[K in keyof Required<T>]: T[K];
}) => {}

export function test2<T = Record<string, never>>(schema: {
>test2 : <T = Record<string, never>>(schema: { [K in keyof Required<T>]: T[K]; }) => void
>schema : { [K in keyof Required<T>]: T[K]; }

[K in keyof Required<T>]: T[K];
}) {};

2 changes: 1 addition & 1 deletion tests/baselines/reference/mappedTypeUnionConstraintInferences.js
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Expand Up @@ -38,7 +38,7 @@ export declare type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>;
export declare type PartialProperties<T, K extends keyof T> = Partial<Pick<T, K>> & Omit<T, K>;
export declare function doSomething_Actual<T extends {
prop: string;
}>(a: T): PartialProperties<T, "prop"> extends infer T_1 ? { [P in keyof T_1]: PartialProperties<T, "prop">[P]; } : never;
}>(a: T): { [P in keyof PartialProperties<T, "prop">]: PartialProperties<T, "prop">[P]; };
export declare function doSomething_Expected<T extends {
prop: string;
}>(a: T): {
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13 changes: 13 additions & 0 deletions tests/cases/compiler/mappedTypeGenericInstantiationPreservesInlineForm.ts
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@@ -0,0 +1,13 @@
// @strict: true
// @declaration: true
// @emitDeclarationOnly: true

// repro from #53109

export const test1 = <T = Record<string, never>>(schema: {
[K in keyof Required<T>]: T[K];
}) => {}

export function test2<T = Record<string, never>>(schema: {
[K in keyof Required<T>]: T[K];
}) {};