Generic type is fully incompatible with conditional type

[mwiencek]

Flow version: 0.278.0

Expected behavior

function fn<T: string>(str: T): void {
    str as (T extends string ? T : T); // works
    str as (T extends number ? T : T); // should work but errors
}

Actual behavior

3:     str as (T extends number ? T : T); // should work but errors
       ^ Cannot cast `str` to conditional type because string [1] is incompatible with conditional type [2]. [incompatible-cast]
    References:
    1: function fn<T: string>(str: T): void {
                                   ^ [1]
    3:     str as (T extends number ? T : T); // should work but errors
                   ^ [2]

I understand this is a very contrived example, but I was trying to come up with a minimal reproduction that demonstrates the issue clearly. We've hit this in real code, and I used something closer to a real example in the Try-Flow below.

• Link to Try-Flow or Github repo: Try-Flow