Implicit search does not work for union types

[scf37]

Compiler version

3.0.0-RC1

Minimized code & Output

https://scastie.scala-lang.org/uvY5sVNcSJuNGBiMPM5zKQ

Expectation

Successful compilation but got an error

[odersky]

It does work if you make Null a separate class, or you compile with -Yexplicit-nulls.

The way things are Null is a bottom type. So every class instance of Optional[C] is in fact C.

I tried to change the definition of Optional to

type Optional[A] = A

then the implicit is not found either. So the problem looks like not a problem with union types at all. If Optional[A] defines a real union it works. It looks rather like a limitation of HK type inference that it cannot infer identities. That's actually expected, I think.

[odersky]

I am closing this. Reopen if someone has an argument that identities should be inferrable for HK types and how to do it.