How to extract a higher-kinded type in a quoted pattern?

[julienrf]

Consider the following macro definition:

import scala.quoted.*

trait Foo:
  type Bar
  type Baz[A]

inline def macroDef(foo: Foo) = ${ macroImpl('foo) }

def macroImpl(foo: Expr[Foo])(using Quotes): Expr[Unit] =
  foo match
    case '{ $ignored: Foo { type Bar = bar } } =>
      quotes.reflect.report.info(s"foo.Bar = ${Type.show[bar]}")
  foo match
    case '{ $ignored: Foo { type Baz[A] = baz } } =>
      quotes.reflect.report.info(s"foo.Baz[Int] = ${Type.show[baz[Int]]}")
  '{ () }

The first match expression does work and prints something during the macro expansion, however, the second match expression does not even compile, it says:

[error] 140 |        quotes.reflect.report.info(s"foo.Baz[Int] = ${Type.show[baz[Int]]}")
[error]     |                                                                ^^^^^^^^
[error]     |                                     baz does not take type parameters

---

I’ve tried different variations, without success:

    case '{ $ignored: Foo { type Baz[A] = baz } } =>
      quotes.reflect.report.info(s"foo.Baz = ${Type.show[baz]}")

This one compiles but does not match at macro-expansion-time.

---

    case '{ type baz[A]; $ignored: Foo { type Baz[A] = `baz`[A] } } =>
      quotes.reflect.report.info(s"foo.Baz[Int] = ${Type.show[baz[Int]]}")

This one does not compile:

[error] 139 |      case '{ type baz[A]; $ignored: Foo { type Baz[A] = `baz`[A] } } =>
[error]     |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]     |              Missing type parameter for baz

[nicolasstucki]

We are still missing support for higher-kinded type holes