Conditional types can't resolve `keyof this`

[falsandtru]

cc @ahejlsberg

TypeScript Version: 2.7.0-dev.20180210

Search Terms:

Code

type OldDiff<T extends string, U extends string> = (
  & { [P in T]: P; }
  & { [P in U]: never; }
  & { [x: string]: never; }
)[T];
type NewDiff<T, U> = T extends U ? never : T;
interface A {
  a: 'a';
}
interface B1 extends A {
  b: 'b';
  c: OldDiff<keyof this, keyof A>;
}
interface B2 extends A {
  b: 'b';
  c: NewDiff<keyof this, keyof A>;
}
type c1 = B1['c']; // 'c' | 'b'
type c2 = B2['c']; // 'c' | 'a' | 'b'

Expected behavior:

c2 is 'c' | 'b'.

Actual behavior:

c2 is 'c' | 'a' | 'b'.

Playground Link:

Related Issues:

[falsandtru]

@ahejlsberg Is this not a bug?

[ahejlsberg]

@falsandtru Don't know yet, haven't had time to look.

[falsandtru]

I see.