Inferences from type aliases disregard variance #31659
Description
TypeScript Version: 3.5.0 and previous
Code
// @strictFunctionTypes: true
type Func<T> = (x: T) => void;
declare let f1: Func<string>;
declare let f2: Func<"a">;
declare function foo<T>(f1: Func<T>, f2: Func<T>): void;
foo(f1, f2); // ErrorExpected behavior:
No error.
Actual behavior:
Error.
When inferring between instantiations of the same type alias we always make covariant inferences instead of computing and using proper variance information.
Activity
jcalz commentedon May 29, 2019
Possibly duplicate of or related to #31251 and #31295?
ahejlsberg commentedon May 29, 2019
No, I don't think either of those are related.
jcalz commentedon May 29, 2019
Okay (Joe Calzaretta (@jcalz) slinks away)