Mixing macros doesn't work if the scala 2 macro needs a WeakTypeTag of a type parameter to an enclosing class

[coreywoodfield]

Compiler version

3.2.1

Minimized code

import scala.language.experimental.macros
import scala.quoted.{Quotes, Expr, Type}

class Foo[A]
object Foo {
  def apply[A] = new Foo[A]
}

trait TraitWithTypeParam[A] {
  protected inline def foo: Foo[A] = ${ MacrosImpl.fooImpl[A] }
  protected def foo: Foo[A] = macro MacrosImpl.compatFooImpl[A]
}

object MacrosImpl {
  def fooImpl[A: Type](using quotes: Quotes): Expr[Foo[A]] = '{new Foo[A]}

  def compatFooImpl[A: c.WeakTypeTag](c: scala.reflect.macros.blackbox.Context): c.Tree = {
    import c.universe._
    TypeApply(
      Select(
        Ident(c.mirror.staticModule(classOf[Foo[A]].getName)),
        TermName("apply")
      ),
      List(TypeTree(weakTypeOf[A])),
    )
  }
}

build.sbt:

lazy val macroImpl = project.in(file("macro-impl"))
  .settings(
    scalaVersion := "3.2.1",
    libraryDependencies ++= Seq(
      "org.scala-lang" % "scala-reflect" % "2.13.10",
      "org.scala-lang" %% "scala3-compiler" % scalaVersion.value % Provided,
    )
  )

Output

[error] -- Error: /home/cwoodfield/lucid/macrotest/macro-impl/src/main/scala/location/Location.scala:11:25 
[error] 11 |trait TraitWithTypeParam[A] {
[error]    |                         ^
[error]    |                Something's wrong: missing original symbol for type tree
[error] -- Error: /home/cwoodfield/lucid/macrotest/macro-impl/src/main/scala/location/Location.scala:12:61 
[error] 12 |  protected inline def foo: Foo[A] = ${ MacrosImpl.fooImpl[A] }
[error]    |                                                             ^
[error]    |No given instance of type quoted.Type[A] was found for an implicit parameter of method fooImpl in object MacrosImpl
[error]    |
[error]    |where:    A is a type in trait TraitWithTypeParam which is an alias of <error Something's wrong: missing original symbol for type tree>
[error] two errors found

Expectation

This should compile and work as described in https://docs.scala-lang.org/scala3/guides/migration/tutorial-macro-mixing.html. Instead it doesn't compile.

Note that moving the [A] type parameter to the methods (i.e.

trait TraitWithTypeParam {
  protected inline def foo[A]: Foo[A] = ${ MacrosImpl.fooImpl[A] }
  protected def foo[A]: Foo[A] = macro MacrosImpl.compatFooImpl[A]
}

) or commenting out the scala 2 macro results in the code compiling. So it seems like attempting to call a scala 2 macro that takes a WeakTypeTag for a type that's a parameter to a class messes up the type tree for the class type parameter somehow

[prolativ]

Another workaround seems to be to make A a type member and add a proxy which has it as a type parameter:

import scala.language.experimental.macros
import scala.quoted.{Quotes, Expr, Type}

class Foo[A]
object Foo {
  def apply[A] = new Foo[A]
}

trait TraitWithTypeMember {
  type A
  protected inline def foo: Foo[A] = ${ MacrosImpl.fooImpl[A] }
  protected def foo: Foo[A] = macro MacrosImpl.compatFooImpl[A]
}

trait TraitWithTypeParam[A0] extends TraitWithTypeMember {
  type A = A0
}

object MacrosImpl {
  def fooImpl[A: Type](using quotes: Quotes): Expr[Foo[A]] = '{new Foo[A]}

  def compatFooImpl[A: c.WeakTypeTag](c: scala.reflect.macros.blackbox.Context): c.Tree = {
    import c.universe._
    TypeApply(
      Select(
        Ident(c.mirror.staticModule(classOf[Foo[A]].getName)),
        TermName("apply")
      ),
      List(TypeTree(weakTypeOf[A])),
    )
  }
}

This compiles however I haven't tested it at use site

[coreywoodfield]

Hmm that does compile, but when you call the macro from scala 2, the WeakTypeTag is just a scala.reflect.internal.Types#AbstractNoArgsTypeRef, the typeSymbol.fullName is just TraitWithTypeMember.A, and nothing I've tried (dealias, resultType, etc) has been able to get any information about the actual type parameter: all it knows is that it's the type member A of TraitWithTypeMember. Changing the : c.WeakTypeTag to : c.TypeTag results in

[error] .../macrotest/test/Test.scala:4:3: exception during macro expansion: 
[error] java.lang.IllegalArgumentException: wrong number of arguments
[error]   foo
[error]   ^
[error] one error found

at the call-site in scala 2. I appreciate the suggestion but it doesn't appear to actually be effective 🙃

[johnduffell]

This is still happening in scala 3.3.1, it's blocking scala-logging to be updated to work on mixed scala 2.13/3 projects.