Macro definition and expansion in same compilation run can cause name collisions with toplevel definitions

[jodersky]

If a macro is expanded by a toplevel function and at another place in the same package but in a different file, and if the macro is defined in the same compilation run, a name collision appears.

Consider the following example. Assume these 3 files:

• the macro definition itself, Macro.scala:

  object Macro {
    inline def expand(): Unit = ${impl}
    def impl(using scala.quoted.QuoteContext) = '{???}
  }
  

• the first expansion site, Clazz.scala:

  class Clazz {
    def foo = Macro.expand()
  }
  

• the second expansion site, toplevel.scala:

  def fct: Unit = Macro.expand()
  

Compiling all three files at once will produce the following error.

dotc Macro.scala Clazz.scala toplevel.scala:

-- Error: toplevel.scala:1:0 ---------------------------------------------------
1 |def fct: Unit = Macro.expand()
  |^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |toplevel$package$ is already defined as package object toplevel$package
1 error found

Some notes:

• compiling them separately works as intended:

  dotc Macro.scala && dotc Clazz.scala toplevel.scala # OK
  dotc Macro.scala Clazz.scala && dotc toplevel.scala # OK
  dotc Macro.scala toplevel.scala && dotc Clazz.scala # OK
  

• Clazz and fct must be defined in the same package but in different files,
  for this bug to manifest itself.

• the other expansion site doesn't have to be in a class. It can very well be another toplevel function in a different file. The issue will manifest itself as long as expansions are in different files but in the same package and at least one of them is a toplevel definition.

• this was observed on version 0.25.0-RC2

• here's an example project with the above mentioned files: https://github.com/jodersky/dotty-bug

/cc @nicolasstucki @manojo

[nicolasstucki]

It looks like the symbol toplevel$package$ from the previous run is kept which causes the conflict.

[odersky]

@nicolasstucki In this example, do we split compilation into multiple runs by suspending some calls?

[nicolasstucki]

Yes. In the first run we suspend Clazz.scala and toplevel.scala, then in the second run there is the conflict with the symbol of the definition of toplevel$package. Possibly a reference to toplevel$package from Clazz.scala.