Narrowing type via Function Assertion loses numeric type

[eyedean]

TypeScript Version: 3.9.0-dev.20200406

Search Terms: assertion, function, narrowing, numeric types

Code

// GOOD EXAMPLE
function f1(x: any): asserts x is number | undefined { }
let v1: number | string | undefined;
f1(v1)
type t1 = typeof v1; // t1 = number | undefined, as excepted

// BUGGY EXAMPLE
function f2(x: any): asserts x is 6 | undefined { }
let v2: number | string | undefined;
f2(v2)
type t2 = typeof v2; // t2 = undefined. Expected to be 6 | undefined

Expected behavior: t2 must be 6 | undefined.

Actual behavior: t2 is undefined only.

Playground Link: Playground Link

Related Issues: I went through the comments of the popular #32695 by Anders Hejlsberg (@ahejlsberg) and also searched in the existing issues but couldn't find any similar problem being reported in the past.

FWIW, I ran into this trying to write a assertIsInEnumValues for Numeric Enums. (related StackOverflow question: https://stackoverflow.com/a/61058457)

[jack-williams]

I think this is a duplicate of #31156

[eyedean]

Jack Williams (@jack-williams) thanks for commenting. It's definitely not limited to undefined (I just tested and it has the same bug with boolean as well). However, I can definitely smell that "branded primitives" discussed at #31206 might have caused it.

[sandersn]

Yep, it's a duplicate of #31156 -- the unions on the source and target are the cause. Swapping undefined for boolean has the same problem.