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Unable to narrow parameterized generic types for functions #25039
Description
TypeScript Version: 2.8
Code
function test<T extends 'a' | 'b'>(x: T) {
if (x === 'a') {
const z1 = x;
}
}
function test2(x: 'a' | 'b') {
if (x === 'a') {
const z2 = x;
}
}Expected behavior:
z1 and z2 should have the same type, which is a
Actual behavior:
function test is unable to narrow parameter x, with the if check, and z2 resolves to x
Activity
[-]Unable to narrow generic types for functions[/-][+]Unable to narrow parameterized generic types for functions[/+]MartinJohns commentedon Jun 18, 2018
With
strictNullChecksenabled your code won't even compile, because the typenullis not compatible with the type'a' | 'b'.Without
strictNullChecksthe valuenullis implicitly compatible with every type. When you calltest(null)the compiler resolves toanybecause it can't figure out the type ofT. Based onnullthe compiler can't deduct any sensible type. You could expect that it should return'a' | 'b', becauseTmust extend from it, but that would still only be a "guess". There's noT-type known.lingz commentedon Jun 18, 2018
I think I communicated the issue confusingly, it was not about the actual return type but the type guard in the implementation, I edited the first post to reflect that.
mhegazy commentedon Jun 19, 2018
Duplicate of #13995
typescript-bot commentedon Jul 5, 2018
Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.