Generic type not narrowed using flow control analysis

[magnushiie]

TypeScript Version: 2.4.0

Code

// A *self-contained* demonstration of the problem follows...

function testUnion() {
    type X = "A" | "B";

    function test(x: X) {
        if (x === "A") {
            const copyA: "A" = x; // succeeds, x's type is "A"
            // const copyB: "B" = x; // error
        }
    }
}

function testExtends() {
    type X = "A" | "B";

    function testZ<Z extends X>(x: Z) {
        if (x === "A") {
            const copyA: "A" = x; // error, x's type is still Z extends "A" | "B"
            // const copyB: "B" = x; // error, as expected
        }
    }
}

Expected behavior:
The initialization of copyA in testExtends should succeed.

Actual behavior:

src/test.ts(17,19): error TS2322: Type 'Z' is not assignable to type '"A"'.
  Type 'X' is not assignable to type '"A"'.
    Type '"B"' is not assignable to type '"A"'.

This prevents some useful generic implementations where the actual type of X is a keyof type expression, and other types are resolved based on the type of x.

[magnushiie]

Ah, this seems to be a duplicate of #13995