Inferences from type aliases disregard variance

[ahejlsberg]

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);  // Error

Expected 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.

[jcalz]

Possibly duplicate of or related to #31251 and #31295?

[ahejlsberg]

Possibly duplicate of or related to #31251 and #31295?

No, I don't think either of those are related.

[jcalz]

Okay (@jcalz slinks away)