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Computing constraint of conditional type by replacing contravariant infer variables with their constraints is unsound #26945

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mattmccutchen opened this issue Sep 6, 2018 · 1 comment
Open

Computing constraint of conditional type by replacing contravariant infer variables with their constraints is unsound #26945

mattmccutchen opened this issue Sep 6, 2018 · 1 comment
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@ahejlsberg
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Bug A bug in TypeScript
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@mattmccutchen
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TypeScript Version: master (8f654f0)

Search Terms:
conditional type assignability assignable constraint contravariant infer variable parameter unsound

Code

type Foo<A> = [A] extends [[infer B]] ? (arg: B) => void : never;
function fortyTwo<A>(x: Foo<A>) { x(42); }
fortyTwo<[string]>((arg: string) => console.log(arg.slice()));

Expected behavior: Compile error on x(42).

Actual behavior: Successful compilation and runtime error.

Playground Link: link

Related Issues: #26933
@mattmccutchen mattmccutchen mentioned this issue Sep 6, 2018
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@jack-williams
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What would be the fix for this, substituting the lower bound (never) instead, or just failing?

@RyanCavanaugh RyanCavanaugh added this to the Future milestone Sep 7, 2018
@RyanCavanaugh RyanCavanaugh added the Bug A bug in TypeScript label Sep 7, 2018
@dgreensp dgreensp mentioned this issue Jan 8, 2020
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@ahejlsberg ahejlsberg mentioned this issue Jan 11, 2022
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