Mapped types intersections are too eagerly resolved

[gcnew]

TypeScript Version: nightly (2.5.0-dev.20170727)

Code

type StringContains<S extends string, L extends string> = (
 { [K in S]: 'true' } &
 { [key: string]: 'false' }
 )[L]

type ObjectHasKey<O, L extends string> = StringContains<keyof O, L>

type First<T> = ObjectHasKey<T, '0'> // eagerly resolved as 'false'

Expected behavior:
Type First should be left as an indexing operation.

PS: origin of the issue: gcanti/typelevel-ts#8

[SimonMeskens]

Anders Hejlsberg (@ahejlsberg) Could you have a look at this one too?