[BUG] union type generic extends child class with generic method

TypeScript Version: all, tested from 3.3 to 4.2

Search Terms: union type generic extends child class with generic method

Code

class A<GValue> {
  a<GKey extends GValue>(value: GKey): void {}
}

class B<GValue> extends A<GValue> {
  a<GKey extends GValue>(value: GKey): void {}
}


const a1: (('a' | 'b') extends any ? true : false) = true;
const a2: (('a') extends any ? true : false) = true;

const a3: (B<'a' | 'b'> extends A<any> ? true : false) = true; // error before v3.7
const a4: (B<'a'> extends A<any> ? true : false) = true; // error
const a5: (B<'a'> extends B<any> ? true : false) = true;

type Constrained<G extends A<any>> = true;

const a6: Constrained<A<'a'>> = true; // ok
const a7: Constrained<B<'a' | 'b'>> = true; // ok
const a8: Constrained<B<'a'>> = true; // error

Details:

• a1 and a2 shows us that a string or an union of string extends any => legit

• a3 shows us that it's still true, except before typescript 3.7 => true seems legit

• a4 creates an error because (B<'a'> extends A<any> ? true : false) is actually false => bug, true expected.
  So, an union is true, but not a single value.

• a5 is true which is correct

• a6, a7 and a8 shows the same but with a type constraint

Notes:

• a3 has not the same behavior between before and after v3.7
• the bug appears only when we have a generic type on a method which extends another generic type coming from the class

Playground Link: playground link

Related Issues: this bug is pretty hard to reach and to describe, I was not able to find any related issue.
But a good start could be to search from a fix or update from the 3.6 to the 3.7, as a3 has not the same behavior between these versions.

提供されている TypeScript の repro と Playground リンクから始め、提案されているとおり 3.6 と 3.7 のコンパイラーの動作を比較します。A を拡張する B のジェネリックメソッドについて、型チェックの経路を追跡します。a4 と a8 のケースが true を受け入れ、既存の a5、a6、a7 の動作が正しいままであれば完了です。