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5.0: const-like inference is not preserved unless a mapped type is usedΒ #53307
Description
Bug Report
π Search Terms
const type, validator type
π Version & Regression Information
5.0.2
- I was unable to test this on prior versions because const type parameters are a new feature
β― Playground Link
Playground link with relevant code
π» Code
type NotEmpty<T extends Record<string, any>> = keyof T extends never ? never : T;
const thing = <const O extends Record<string, any>>(o: NotEmpty<O>) => {
return o;
};
const t = thing({foo: ''});
// is inferred as { foo: string } β
// BUT, when adding a mapped type to NotEmpty it works
type NotEmptyMapped<T extends Record<string, any>> = keyof T extends never ? never : {
[K in keyof T]: T[K] // β¬
οΈ
};
const thingMapped = <const O extends Record<string, any>>(o: NotEmptyMapped<O>) => {
return o;
};
const tMapped = thingMapped({foo: ''});
// is inferred as { foo: "" } β
π Actual behavior
When using a non-mapped validator type, the "valid" branch does not preserve the const-like inference unless the type is mapped over.
π Expected behavior
A mapped type is not needed to preserve const-like inference.
Activity
Andarist commentedon Mar 17, 2023
Working case:
{ [K in keyof O]: O[K]; }O["foo"]isConstTypeParameterContextis able to returntruebased on this informationNot working case:
Record<string, any>anyisConstTypeParameterContextreturnfalsebased on thatconorbrandon commentedon Mar 17, 2023
Interesting! Thanks for taking a look.
I did find it a bit curious that
NotEmptyMappedunfortunately doesn't seem to actually error for an empty object, and instead returnsRecord<string, any>. Is that possibly related?Both correctly error if the check is instead changed to
{} extends T, but have the same const-like inference mismatch.Finally if the check is
T extends Record<string, never>, both don't use const-like inference andNotEmptyMappeddoesn't error on an empty object, butNotEmptydoes.I think that's all the ways I can think of to check for an empty object...
ahejlsberg commentedon Mar 17, 2023
It looks like we need to probe types a little deeper in
isConstTypeVariable.