Unhelpful "Tree had an unexpected owner" error message

[kubukoz]

Compiler version

3.2.1

Minimized code

//> using scala "3.2.1"
//> using option "-Ycheck:all"
//> using option "-Xcheck-macros"
import scala.annotation.experimental

import scala.quoted.*

object testmacros {
  inline def example = ${ exampleImpl }

  @experimental
  def exampleImpl(using Quotes): Expr[Any] = {
    import quotes.reflect._

    val methodType = MethodType(List("arg"))(_ => List(TypeRepr.of[String]), _ => TypeRepr.of[Unit])
    def method(
      owner: Symbol
    ) = Symbol.newMethod(
      owner,
      "test",
      methodType,
    )

    val clazz = Symbol.newClass(
      Symbol.spliceOwner,
      "$anon",
      List(TypeRepr.of[Object]),
      owner => method(owner) :: Nil,
      None,
    )

    ClassDef(
      clazz,
      Nil,
      List(
        DefDef(
          method(clazz),
          argss =>
            Some(
              Lambda(method(clazz), methodType, (ls, _) => '{ ??? }.asTerm)
            ),
        )
      ),
    ).asExpr
  }

}

//in another file

val f = testmacros.example

Output

...
[error] Expected: method test (demo$._$_$$anon.test)
[error] But was: method test (demo$._$_$$anon.test)
...

Full output

[error] ./main.scala:46:3: Exception occurred while executing macro expansion.
[error] java.lang.AssertionError: assertion failed: Tree had an unexpected owner for method $anonfun
[error] Expected: method test (demo$._$_$$anon.test)
[error] But was: method test (demo$._$_$$anon.test)
[error] 
[error] 
[error] The code of the definition of method $anonfun is
[error] def $anonfun(arg: java.lang.String): scala.Unit = scala.Predef.???
[error] 
[error] which was found in the code
[error] ((arg: java.lang.String) => scala.Predef.???)
[error] 
[error] which has the AST representation
[error] Block(List(DefDef("$anonfun", List(TermParamClause(List(ValDef("arg", Inferred(), None)))), Inferred(), Some(Inlined(Some(TypeIdent("macros$")), Nil, Ident("???"))))), Closure(Ident("$anonfun"), None))
[error] 
[error] 
[error] 
[error] Tip: The owner of a tree can be changed using method `Tree.changeOwner`.
[error] Tip: The default owner of definitions created in quotes can be changed using method `Symbol.asQuotes`.
[error] 
[error]         at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
[error]         at scala.quoted.runtime.impl.QuotesImpl$$anon$9.traverse(QuotesImpl.scala:2949)
[error]         at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1657)
[error]         at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1657)
[error]         at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.fold$1(Trees.scala:1529)
[error]         at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.apply(Trees.scala:1531)
[error]         at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1562)
[error]         at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1658)
[error]         at scala.quoted.runtime.impl.QuotesImpl$$anon$9.traverse(QuotesImpl.scala:2956)
[error]         at scala.quoted.runtime.impl.QuotesImpl$reflect$.xCheckMacroOwners(QuotesImpl.scala:2957)
[error]         at scala.quoted.runtime.impl.QuotesImpl$reflect$.scala$quoted$runtime$impl$QuotesImpl$reflect$$$xCheckMacroedOwners(QuotesImpl.scala:2915)
[error]         at scala.quoted.runtime.impl.QuotesImpl$reflect$DefDef$.apply$$anonfun$3$$anonfun$1(QuotesImpl.scala:272)
[error]         at dotty.tools.dotc.ast.tpd$.DefDef(tpd.scala:289)
[error]         at scala.quoted.runtime.impl.QuotesImpl$reflect$DefDef$.apply$$anonfun$3(QuotesImpl.scala:273)
[error]         at scala.quoted.runtime.impl.QuotesImpl$reflect$.scala$quoted$runtime$impl$QuotesImpl$reflect$$$withDefaultPos(QuotesImpl.scala:2906)
[error]         at scala.quoted.runtime.impl.QuotesImpl$reflect$DefDef$.apply(QuotesImpl.scala:273)
[error]         at scala.quoted.runtime.impl.QuotesImpl$reflect$DefDef$.apply(QuotesImpl.scala:269)
[error]         at macros$.exampleImpl(macros.scala:182)
[error] 
[error]   macros.example
[error]   ^^^^^^^^^^^^^^
Error compiling project (Scala 3.2.1, JVM)

Expectation

Something that distinguishes the actual owner from the expected one.

[kubukoz]

Sometimes I'm even seeing this:

Error: Unexpected error when compiling project_15da074c1d: 'assertion failed: bad owner; method $anonfun has owner method hello, expected was method hello

[nicolasstucki]

There is a bug in the maco implementation. The code instantiates 3 times a new symbol for the same methods (method(owner), method(clazz), and method(clazz)). As these 3 new symbols have the same name, the error message shows the same name.

To fix the macro implementation one needs to the method symbol once when creating the class symbol. Then that symbol can be recovered using methodMember.

def method(owner: Symbol) = Symbol.newMethod(..., "test", ...)
val clazz = Symbol.newClass(...)
val testSym = clazz.methodMember("test").head
ClassDef(
  clazz,
  Nil,
  List(
    DefDef(
      testSym,
      argss =>
        Some(
          Lambda(testSym, methodType, (ls, _) => {
            '{ ??? }.asTerm
          })
        )
    )
  ),
)

[kubukoz]

@nicolasstucki I'm aware the macro may be buggy, the problem I was reporting is just the... reporting :)

[nicolasstucki]

The description of the error was meant as an in depth analysis of the issue that can be compared to the new hint that will be reported when such an error occurs. Anyone else having a look at this issue would not necessarily have this insight.

[kubukoz]

I see, thank you.