tests/cases/compiler/genericImplements.ts(9,5): error TS2416: Property 'f' in type 'X' is not assignable to the same property in base type 'I'.
  Type '<T extends B>() => T' is not assignable to type '<T extends A>() => T'.
    Type 'B' is not assignable to type 'T'.
      'T' could be instantiated with an arbitrary type which could be unrelated to 'B'.


==== tests/cases/compiler/genericImplements.ts (1 errors) ====
    class A { a; };
    class B { b; };
    interface I {
        f<T extends A>(): T;
    } // { f: () => { a; } }
    
    // OK
    class X implements I {  
        f<T extends B>(): T { return undefined; }
        ~
!!! error TS2416: Property 'f' in type 'X' is not assignable to the same property in base type 'I'.
!!! error TS2416:   Type '<T extends B>() => T' is not assignable to type '<T extends A>() => T'.
!!! error TS2416:     Type 'B' is not assignable to type 'T'.
!!! error TS2416:       'T' could be instantiated with an arbitrary type which could be unrelated to 'B'.
    } // { f: () => { b; } }
    
    // OK
    class Y implements I {
        f<T extends A>(): T { return undefined; }
    } // { f: () => { a; } }
    
    // OK
    class Z implements I {
        f<T>(): T { return undefined; }
    } // { f: <T>() => T } 