Narrowed type of variable isn't obtained by aliases of the variable

[sandersn]

function f() {}
f.p = { x: "" };
const p = f.p;
if (1 + 1 < 2) {
    f.p = { x: 0 };
}
f.p.x = 0;
p.x.toUpperCase(); // No compile error, crash at runtime

Expected behavior:

Error on p.x.toUpperCase()

Actual behavior:
No error.

[sandersn]

Unfortunately, this bug repros with any variable:

var ex: { x: string } | { x: number } = { x: "" }
const p = ex
if (1 + 1 < 2) {
    ex = { x: 0 }
}
ex.x = 0
p.x.toUpperCase();

[sandersn]

@ahejlsberg I believe this bug exposes a design limitation of the control flow algorithm that we don't intend to fix. Can you confirm?

[RyanCavanaugh]

Aliasing effects are definitely missed