kind-projector's placeholder syntax not working for infix type constructors

[TomasMikula]

Compiler version

3.7.4

Minimized code

https://scastie.scala-lang.org/VJbhRJdSSaKJFBn8Yod7HA

trait ::[A, B]

def foo[F[_]] = ()

foo[::[Int, _]] // OK
foo[::[_, Int]] // OK
foo[Int :: _]   // error: Not found: type _
foo[_ :: Int]   // error: Not found: type _

Output

Not found: type _
Not found: type _

Expectation

I'd expect the infix notation work the same as the prefix notation.

[xuwei-k]

• -Ykind-projector does not support infix type lampepfl/dotty-feature-requests#117
• -Ykind-projector does not support infix type #17896

[TomasMikula]

Thanks, @xuwei-k, for linking to related discussions.

If the functionality of -Ykind-projector:underscores is not planned to be extended, my question is, when will Scala support the _ syntax for type lambdas, as advertised in Kind Projector Migration:

In the future, Scala 3 will use the _ underscore symbol for placeholders in type lambdas—just as the underscore is currently used for placeholders in (ordinary) term-level lambdas.

[neko-kai]

@TomasMikula Martin's latest answer was as soon as someone from the community takes the lead on this. Now who that might be is the tricky part.