Type inference for string literal union

[Galfour]

TypeScript Version: 1.8.0 / nightly (2.0.0-dev.201xxxxx)

type dis = 'a' | 'b' | 'c';
type Cmap = Map<dis, number>;
var a: Cmap = new Map([['a', 2], ['b', 3]]);

type dis = 'a' | 'b' | 'c';
let a = 'a';
let b:dis = a;

Expected behavior:
inferred type of a is Map<dis, number>
inferred type of b is dis

Actual behavior:
inferred type of a is Map<string, number>
inferred type of b is string

Hello !
There is no map notation, so we have to use the Map constructor. However, it is not correctly typed, and requires type annotation () at every construction.
EDIT : The problem is not with Map. Cf, the other example.
It looks like the string union type is not propagated.
I found it to be expected when a is explicitely typed as 'string'. But when no annotation is given, I did not find the expected semantic.
As such, I do not know if it is a bug, or by design, but I did not find anything referring to that particular issue.

Have a good day !

[DanielRosenwasser]

Hey @Galfour, thanks for filing, but this looks like a duplicate of #9489. We can continue the conversation there.