given x : T, bool should be a subtype of x is T

[acertain]

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)

[aluanhaddad]

This looks like a duplicate of #10734

[ghost]

There's no type guard for us to infer in the not-working case.
x > 3 isn't correctly of type x is number, because 2 is a number but it would return false. When you explicitly annotate a type predicate we trust you and allow it, but inferring one in this case would be wrong.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.