Generic type param used as generic type constraint causes variance issues?

[AnyhowStep]

TypeScript Version: 3.5.1, 3.6.3, Nightly

Search Terms:

generic type parameter, generic type constraint, variance

Code

declare const x0 : <InT extends boolean|null>(arg : InT) => boolean;
//Expected: Error
//Actual : Error
const x1 : <InT extends string>(arg : InT) => boolean = x0;

type Y<In, Out> = <InT extends In>(arg : InT) => Out;
declare const y0 : Y<boolean|null, boolean>;
//Expected: Error
//Actual : OK
const y1 : Y<string, boolean> = y0;

Expected behavior:

y1 = y0 assignment should fail

Actual behavior:

y1 = y0 assignment passes

Playground Link:

3.5.1 Playground

3.6.3 Playground

Nightly Playground

Related Issues:

[AnyhowStep]

Seems like the type inference is OK, though,

/**
 * type test0 = [boolean | null, boolean]
 */
type test0 = typeof y0 extends Y<infer In, infer Out> ?
 [In, Out] :
 never;

/**
 * type test1 = [string, boolean]
 */
type test1 = typeof y1 extends Y<infer In, infer Out> ?
 [In, Out] :
 never;

3.5.1 Playground

[jack-williams]

Duplicate of #31006 and fixed by #31029.

[AnyhowStep]

It's not in the nightly builds?

[Edit]
Oh, not merged yet.

I guess I'll close this and monitor that PR. Thanks!