=== tests/cases/compiler/classImplementsClass3.ts ===
class A { foo(): number { return 1; } }
>A : A
>foo : () => number
>1 : number

class C implements A {
>C : C
>A : A

    foo() {
>foo : () => number

        return 1;
>1 : number
    }
}

class C2 extends A {}
>C2 : C2
>A : A

// no errors
var c: C;
>c : C
>C : C

var c2: C2;
>c2 : C2
>C2 : C2

c = c2;
>c = c2 : C2
>c : C
>c2 : C2

c2 = c;
>c2 = c : C
>c2 : C2
>c : C