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given x : T, bool should be a subtype of x is T #19557
Description
If x : T is already known, any bool implies/is evidence for x is T.
This should help type inference for e.g.
function f<T, R extends T>(x: T[], p: (x: T) => x is R): R | undefined {
for (const v of x) {
if(p(v)){return v}
}
}
// doesn't work
f([2,3,4], (x) => x > 3)
// works
f([2,3,4], (x): x is number => x > 3)
(filter, etc)
Activity
[-]given `x : T`, `bool` should be a subtype of `x is T`[/-][+]given x : T, bool should be a subtype of x is T[/+]aluanhaddad commentedon Oct 30, 2017
This looks like a duplicate of #10734typescript-bot commentedon Nov 14, 2017
Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.