Generic methods with type parameters in generic constraints cannot be overridden

[jcalz]

TypeScript Version: 3.0.0-dev.20180630

Search Terms: generic method, subclass, override, type parameter constraint

Code

class A {
    m<T, U extends T>(t: T, u: U) { }
}

class B extends A {
    m<T, U extends T>(t: T, u: U) { } 
//  ﹋ error
}

Expected behavior:
No errors; the m method in the subclass has exactly the same signature as the method it is overriding.

Actual behavior:
The compiler warns on the method override B.m:

Type '<T, U extends T>(t: T, u: U) => void' is not assignable 
  to type '<T, U extends T>(t: T, u: U) => void'.
Two different types with this name exist, but they are unrelated.
Types of parameters 'u' and 'u' are incompatible.
Type 'U' is not assignable to type 'T'.
Type 'T' is not assignable to type 'T'. 
Two different types with this name exist, but they are unrelated.

Note that even if you change the names of the type parameters in method B.m, as in:

class B extends A {
  m<T2, U2 extends T2>(t: T2, u: U2) { }
}

you still get the error Type 'T' is not assignable to type 'T'. Two different types with this name exist, but they are unrelated.

If you remove the extends T generic constraint, it compiles.

Playground Link:
here

Related Issues:
Nothing obviously related; #23960 maybe?

---

So, is this a compiler bug? Or is it intended, and if so, what's the reasoning for it? This issue is inspired by a Stack Overflow question that left me scratching my head.

[RyanCavanaugh]

Worked in 2.4, error introduced in 2.5

[npenin]

any update on this one ? it breaks my CD since more than 1 year because of this bug.

[npenin]

@RyanCavanaugh Any plan to fix this issue ? There might be a workaround using ts-ignore feature, but the ts-ignore comment is not ported over declaration files