CFA doesn't work with generic constraint type

[cevek]

TypeScript Version: 3.4.0-dev.20190307

Search Terms:

Code

function foo<K extends 'a' | 'b'>(k: K) {
  if (k === 'a') {
     const x: 'a' = k; // Type 'K' is not assignable to type '"a"
  }
}

Expected behavior:

Actual behavior:

Playground Link:

Related Issues:

[jack-williams]

Related: #20375