Type indexing with string literals starting with __ does not work

[guncha]

TypeScript Version: 2.3.0-dev.20170223

Code

It's very likely that this is already reported, but I couldn't find it after a quick search through the issues.

type Foo = {
  bar: string
  __typename: string
}

type A = Foo["bar"] // ok 
type B = Foo["__typename"] // Property '__typename' does not exist on type 'Foo'.

Expected behavior:

Type B should be string.

Actual behavior:

Error is thrown.

[guncha]

I did find an older similar issue: #8614

Lo and behold:

type B = Foo["___typename"] // three underscores, ok

[weswigham]

This should be fixed by #16915