Wrong type inference from lambda function's parameter

[KuceraMartin]

Compiler version

Scala 3.3.0

Minimized code

trait Box[T]

def f[T, U](c: T => T & U)(using Box[T]): Unit = ???

def g[T, U](using Box[T]): Unit =
  def c(t: T) = t.asInstanceOf[T & U]
  f(c)

Output

[error] ./inference.scala:7:7
[error] No given instance of type Box[T & U] was found for parameter x$2 of method f
[error]   f(c)
[error]       ^

Note that the implicit Box[T] is only used to showcase a compilation failure. The real problem is that when calling f from g, the type parameters get inferred as f[T & U, T & U] as can be seen after the typer phase:

def g[T >: Nothing <: Any, U >: Nothing <: Any](using x$1: Box[T]): Unit =
  {
    def c(t: T): T & U = t.asInstanceOf[T & U]
    f[T & U, T & U](
      {
        def $anonfun(t: T): T & U & (T & U) = c(t)
        closure($anonfun)
      }
    )(/* missing */summon[Box[T & U]])
  }

Expectation

The first type parameter should be inferred as T and thus the code should compile.

[som-snytt]

Note f(c(_)) works.

[dwijnand]

Reverted in #19189