[jiaweihli]

TypeScript supports disjoint unions: http://blogs.msdn.com/b/typescript/archive/2015/09/16/announ...

The two real pain points (among those you mentioned) of TypeScript are bivariant function parameters and structural classes, since they lead to potentially unsound code that compiles.

[samwgoldman]

The link you provided doesn't seem to show disjoint union support. I'm talking about this: http://flowtype.org/blog/2015/07/03/Disjoint-Unions.html