Match type reduction fails, but works with minor tweaks

[KuceraMartin]

Compiler version

3.2.2

Minimized code

class C[T]

type Foo[T] = T match
  case C[true] => true
  case C[false] => false

class W[T] extends C[Foo[T]]

def f[T <: C[?]](t: T) = W[T]()

val b = C[true]()

f(b) : C[true]

https://scastie.scala-lang.org/MPU8IU46Q1GRYjcYu13S1g

Output

Compilation error:

Match type reduction failed since selector  T matches none of the cases
    case C[(true : Boolean)] => (true : Boolean)
    case C[(false : Boolean)] => (false : Boolean)

Expectation

The code should compile. In fact there are many small tweaks that make it compile. For example:

• adding case _ in the match type (even though this case is not used in the end): https://scastie.scala-lang.org/U7Tlij41Q6e9D28zxCJlZA
• removing the upper bound T <: C[?] in f: https://scastie.scala-lang.org/tVg5WBVbTlKVsv9v7PLHOw
• storing f(b) into a val: https://scastie.scala-lang.org/tKqK9MMBQ6SNnwmmftuBlw
• explicitly writing the type parameter of f when calling it: https://scastie.scala-lang.org/TyLKSgn9QISZDPiyfoqj0Q

[scala-center-bot]

This issue was picked for the Issue Spree No. 27 of 07 March 2023 which takes place in a week from now. @dwijnand, @mbovel, @jan-pieter, @nmcb will be working on it. If you have any insight into the issue or guidance on how to fix it, please leave it here.

[Decel]

It's a regression, bisection seems to point to 5bf38d8

[dwijnand]

Also 509727a