Check inferred constraints for 'infer X' type variables#22323
Merged
ahejlsberg merged 4 commits intomasterfrom Mar 5, 2018
Merged
Check inferred constraints for 'infer X' type variables#22323ahejlsberg merged 4 commits intomasterfrom
ahejlsberg merged 4 commits intomasterfrom
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mhegazy
approved these changes
Mar 5, 2018
|
@ahejlsberg is there a way to express type Not<T, U> = ... // T can be anything but U, if T is U then error
// use case:
function notSupposedToTakeAsyncCallbacks<X>(callback: () => Not<X, Promise<any>>) {
} |
Contributor
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there is not really a way to throw an error. so the best that is possible today is to return a type that will cause a down-stream error, something like type Not<T, U> = T extends U ? never : T;
type t1 = Not<{ a: number }, Promise<any>>; // { a: number }
type t2 = Not<Promise<number>, Promise<any>>; // never |
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another silly questions please |
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With this PR we properly check inferred constrains for
infer Xtype variables in conditional types. Specifically, during type inference in a conditional type, when a candidate type for aninfer Xtype variable doesn't satisfy the constraint inferred for that type variable (see #21709), the candidate is replaced with the constraint. That will subsequently cause the conditional check to be false. For example:Previously,
T2would be the type42which doesn't satisfy the constraintstringinferred fromMustBeString.Fixes #22221.