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Narrowing type via Function Assertion loses numeric type #37807
Description
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 | undefinedExpected 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)
Activity
jack-williams commentedon Apr 6, 2020
I think this is a duplicate of #31156
eyedean commentedon Apr 6, 2020
Jack Williams (@jack-williams) thanks for commenting. It's definitely not limited to
undefined(I just tested and it has the same bug withbooleanas well). However, I can definitely smell that "branded primitives" discussed at #31206 might have caused it.sandersn commentedon Apr 7, 2020
Yep, it's a duplicate of #31156 -- the unions on the source and target are the cause. Swapping
undefinedforbooleanhas the same problem.