Crash when refining a private type member

[EugeneFlesselle]

Compiler version

3.4.2-RC1

Minimized code

trait A:
  private type M

def foo(a: A{type M = Int}) =
  val _: a.M = ??? // crash

Note that replacing ??? with 0 gives an error instead.

Output (click arrow to expand)

  exception while retyping val _$1: a.M = ??? of class ValDef # -1

  An unhandled exception was thrown in the compiler.
  Please file a crash report here:
  https://github.com/scala/scala3/issues/new/choose
  For non-enriched exceptions, compile with -Yno-enrich-error-messages.

     while compiling: tests/playground/example.scala
        during phase: MegaPhase{elimErasedValueType, pureStats, vcElideAllocations, etaReduce, arrayApply, elimPolyFunction, tailrec, completeJavaEnums, mixin, lazyVals, memoize, nonLocalReturns, capturedVars}
                mode: Mode(ImplicitsEnabled)
     library version: version 2.13.12
    compiler version: version 3.4.2-RC1-bin-SNAPSHOT-nonbootstrapped-git-3694d95
            settings: -classpath /Users/eugeneflesselle/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.13.12/scala-library-2.13.12.jar:/Users/eugeneflesselle/Documents/dotty/library/../out/bootstrap/scala3-library-bootstrapped/scala-3.4.2-RC1-bin-SNAPSHOT-nonbootstrapped/scala3-library_3-3.4.2-RC1-bin-SNAPSHOT.jar -d /
cannot resolve reference to type (a : A{type M = Int}).M
the classfile defining the type might be missing from the classpath

[odersky]

The fixing commit has been merged since. I think it was on another PR.

[EugeneFlesselle]

The fixing commit has been merged since. I think it was on another PR.

The issue was fixed in #19983, but I do not think it included your commit. Should I just add a regression test or should those changes be merged ?

[EugeneFlesselle]

The test case no longer crashes and does not give an error either.

I expected it to mean we are refining the trait A with an unrelated public type member M, which would be ok, even if non-obvious. But that does not seem to be the case:

type Aux[m] = A {type M = m}
type Leak[a] = a match
  case Aux[m] => m

trait A:
  private type M
  def m: Leak[this.type]

def foo(a: A{type M = Int}): Int = a.m

compiles.

But should it ? I expected something like a non-private method refers to a private type in its signature error, or Leak[a] does not to conform to Int. @smarter

[smarter]

I expected it to mean we are refining the trait A with an unrelated public type member M, which would be ok, even if non-obvious.

Yes, that would be consistent with how a public def in a subclass can have the same name as a private def, otherwise adding a private definition could break backward compatibility.

[EugeneFlesselle]

Yes, that would be consistent with how a public def in a subclass can have the same name as a private def, otherwise adding a private definition could break backward compatibility.

So it should not compile right ?

[smarter]

Right