property access on unioned objects and string indices

[KiaraGrouwstra]

Although not originally mentioned in the laws of union types (#805), I would generally expect a union of types undergoing a transformation to be equivalent to the result of its constituents undergoing the same transformation (distributive law?).

TypeScript Version: 2.5.0-dev.20170626, presumably any version from the introduction of union types

Code

type x = ({ a: 1 } | { a: 2 })['a'];
// 1 | 2
type y = { [k: string]: 3 }['a'];
// 3
type z = ({ a: 1 } | { a: 2 } | { [k: string]: 3 })['a'];
// ^ Property 'a' does not exist on type ...

Expected behavior:
z: 1 | 2 | 3

Actual behavior:
Property 'a' does not exist on type ...

[KiaraGrouwstra]

I've since found that distributivity is not generally guaranteed with unions. This would fail, for one, for operations such as checking whether a given union contains a certain key.

Nevertheless, I believe the above snippet to intuitively indicate a bug.

[mhegazy]

Duplicate of #21141, should be fixed bu #25307