Cannot create instance of parameterized trait in macro

[cb372]

Apologies if I'm doing something stupid but I think this code should work?

Dotty version is 0.25.0-bin-20200516-9450bf2-NIGHTLY.

Minimized code

// Show.scala
trait Show[T] {
  def show(t: T): String
}

object Show {
  inline def deriveWithMacro[T]: Show[T] = ${ impl[T] }

  import quoted._
  def impl[T](using ctx: QuoteContext, tpe: Type[T]): Expr[Show[T]] =
    '{
      new Show[$tpe] {
        def show(t: $tpe): String = "TODO"
      }
    }
}

// Foo.scala
case class Foo(x: String)

object Bar {
  println(Show.deriveWithMacro[Foo].show(Foo("")))
}

Output

[error] 4 |  println(Show.deriveWithMacro[Foo].show(Foo("")))
[error]   |          ^^^^^^^^^^^^^^^^^^^^^^^^^
[error]   |  object creation impossible, since def show(t: T): String is not defined
[error]   |  (Note that T does not match Foo)
[error]   | This location contains code that was inlined from Show.scala:13
[error]   | This location contains code that was inlined from Show.scala:13
[error] one error found
[error] (muService / Compile / compileIncremental) Compilation failed

Expectation

No compile error, prints TODO

[nicolasstucki]

Indeed, it is a bug.

It can worked around it with

    '{
      type X = $tpe
      new Show[X] {
        def show(t: X): String = "TODO"
      }
    }

[nicolasstucki]

It is only the type in the extends clauses that are not properly spliced

  '{
      type X = T
      new Show[X] {
        def show(t: T): String = "TODO"
      }
    }