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Mapped types intersections are too eagerly resolved #17456
Description
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
Activity
SimonMeskens commentedon Jul 27, 2017
Anders Hejlsberg (@ahejlsberg) Could you have a look at this one too?