Fixed an issue with apparent mapped type keys #57838
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,106 @@ | ||
| keyRemappingKeyofResult.ts(82,3): error TS2322: Type 'string' is not assignable to type 'Extract<keyof { [P in keyof T as T[P] extends string ? P : never]: any; }, string>'. | ||
| keyRemappingKeyofResult.ts(90,3): error TS2322: Type 'string' is not assignable to type 'keyof { [P in keyof T as T[P] extends string ? P : never]: any; }'. | ||
| Type 'string' is not assignable to type 'T[P] extends string ? P : never'. | ||
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| ==== keyRemappingKeyofResult.ts (2 errors) ==== | ||
| const sym = Symbol("") | ||
| type Orig = { [k: string]: any, str: any, [sym]: any } | ||
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| type Okay = Exclude<keyof Orig, never> | ||
| // type Okay = string | number | typeof sym | ||
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| type Remapped = { [K in keyof Orig as {} extends Record<K, any> ? never : K]: any } | ||
| /* type Remapped = { | ||
| str: any; | ||
| [sym]: any; | ||
| } */ | ||
| // no string index signature, right? | ||
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| type Oops = Exclude<keyof Remapped, never> | ||
| declare let x: Oops; | ||
| x = sym; | ||
| x = "str"; | ||
| // type Oops = typeof sym <-- what happened to "str"? | ||
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| // equivalently, with an unresolved generic (no `exclude` shenanigans, since conditions won't execute): | ||
| function f<T>() { | ||
| type Orig = { [k: string]: any, str: any, [sym]: any } & T; | ||
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| type Okay = keyof Orig; | ||
| let a: Okay; | ||
| a = "str"; | ||
| a = sym; | ||
| a = "whatever"; | ||
| // type Okay = string | number | typeof sym | ||
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| type Remapped = { [K in keyof Orig as {} extends Record<K, any> ? never : K]: any } | ||
| /* type Remapped = { | ||
| str: any; | ||
| [sym]: any; | ||
| } */ | ||
| // no string index signature, right? | ||
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| type Oops = keyof Remapped; | ||
| let x: Oops; | ||
| x = sym; | ||
| x = "str"; | ||
| } | ||
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| // and another generic case with a _distributive_ mapping, to trigger a different branch in `getIndexType` | ||
| function g<T>() { | ||
| type Orig = { [k: string]: any, str: any, [sym]: any } & T; | ||
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| type Okay = keyof Orig; | ||
| let a: Okay; | ||
| a = "str"; | ||
| a = sym; | ||
| a = "whatever"; | ||
| // type Okay = string | number | typeof sym | ||
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| type NonIndex<T extends PropertyKey> = {} extends Record<T, any> ? never : T; | ||
| type DistributiveNonIndex<T extends PropertyKey> = T extends unknown ? NonIndex<T> : never; | ||
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| type Remapped = { [K in keyof Orig as DistributiveNonIndex<K>]: any } | ||
| /* type Remapped = { | ||
| str: any; | ||
| [sym]: any; | ||
| } */ | ||
| // no string index signature, right? | ||
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| type Oops = keyof Remapped; | ||
| let x: Oops; | ||
| x = sym; | ||
| x = "str"; | ||
| } | ||
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| // https://github.com/microsoft/TypeScript/issues/57827 | ||
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| type StringKeys<T> = Extract< | ||
| keyof { | ||
| [P in keyof T as T[P] extends string ? P : never]: any; | ||
| }, | ||
| string | ||
| >; | ||
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| function test_57827<T>(z: StringKeys<T>) { | ||
| const f: string = z; | ||
| z = "foo"; // error | ||
| ~ | ||
| !!! error TS2322: Type 'string' is not assignable to type 'Extract<keyof { [P in keyof T as T[P] extends string ? P : never]: any; }, string>'. | ||
| } | ||
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| type StringKeys2<T> = keyof { | ||
| [P in keyof T as T[P] extends string ? P : never]: any; | ||
| }; | ||
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| function h<T>(z: StringKeys2<T>) { | ||
| z = "foo"; // error | ||
| ~ | ||
| !!! error TS2322: Type 'string' is not assignable to type 'keyof { [P in keyof T as T[P] extends string ? P : never]: any; }'. | ||
| !!! error TS2322: Type 'string' is not assignable to type 'T[P] extends string ? P : never'. | ||
| const f: string = z; // ok | ||
| } | ||
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| export {}; | ||
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I find it a bit weird that:
keyof unknownisundefined.getApparentMappedTypeKeys. I'd expect it to be insidegetApparentMappedTypeKeys. Isn't there a case wheregetApparentMappedTypeKeysisneverandsourceMappedKeysshould stay asnever?This doesn't look wrong to me, but I can't yet form a good mental model of what's happening, so I don't know what to think. 😅
I'll revisit in a few days and maybe I'll have a better idea of what this all means.
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If we think about
keyof Tas a set of types that can be safely used to access properties ofTinT[keyof T]thennevermakes sense here.neveris the only type that can be safely used askeyof Tafter we instantiateTasunknown.That's what I started with but mapping
nevertoPropertyKeywould break the other call site wheregetApparentMappedTypeKeysis used. In there, it's used to get the target keys. So this would remove the error below:There was a problem hiding this comment.
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This all has me thinking that the original code that uses
getApparentMappedTypeKeyson the source side is just... wrong?