Description
I stumbled upon a rather odd behavior while trying to combine conditional types with generics. It looks like, even when the produced types are structurally different, it's still possible to assign a values between them.
class T1 { t1_tag = 't1' }
class T2 extends T1 { t2_tag = 't2' }
type X<T> = {
x: T extends T2 ? 'yes' : 'no'
}
let a1: X<T1> = { x: 'no' } // {x: 'yes'} is not valid
let a2: X<T2> = { x: 'yes' } // {x: 'no'} is not valid
// These assignations should not type, but they do
a1 = a2
a2 = a1At first I thought this might be caused by generics being intentionally ignored but that seemed weird, since the types end up being structurally different and checks seem to work for literals, so I though I would report it, just in case.
TypeScript Version: 3.6.2
Search Terms: generics, conditional type, type inheritance
Expected behavior: exclusive structural conditional types derived from generic should not be assignable to each other.
Actual behavior: assignation seems to pass type check even when types are structurally incompatible.
Playground Link: here