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

[mattmccutchen]

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

[jack-williams]

What would be the fix for this, substituting the lower bound (never) instead, or just failing?