ReturnType of function with generic type is unknown

TypeScript Version: 3.9.5

Search Terms: return type of function with generic type unknown

Code

type Func = <T = string>() => T;

type R = ReturnType<Func>;

Expected behavior: type R = string

Actual behavior: type R = unknown

Playground Link: https://www.typescriptlang.org/play/index.html#code/C4TwDgpgBAYgrgOwMZQLxQDwBU1QM7ABOAlggOYB8AFAJRoVRYBQTokUASrhxMHIQizgIGeMgpMgA
P.S. the typescript version of playground is 3.9.1, but this bug still exists in 3.9.5.

Related Issues: #26856

I am pretty sure duplicate of #37181

I am pretty sure duplicate of #37181

Thanks, seems this issue does be duplicated.

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