Compiler panic "?C should be a concrete capture set in this phase."

[marvinborner]

In some larger code base I got the following panic:

[error] ?C should be a concrete capture set in this phase.
[info]   at effekt.util.messages$CompilerPanic$.apply(Messages.scala:37)
[info]   at effekt.util.messages$ErrorReporter.panic(Messages.scala:118)
[info]   at effekt.util.messages$ErrorReporter.panic$(Messages.scala:99)
[info]   at effekt.context.Context.panic(Context.scala:41)
[info]   at effekt.util.messages$ErrorReporter.panic(Messages.scala:119)
[info]   at effekt.core.Transformer$.transform(Transformer.scala:861)
[info]   at effekt.core.Transformer$.transform(Transformer.scala:835)
[info]   at effekt.core.Transformer$.coercing$$anonfun$2(Transformer.scala:911)
[info]   at effekt.core.Transformer$.$anonfun$65(Transformer.scala:828)
[info]   at effekt.core.TransformerOps.withBindings$$anonfun$1(Transformer.scala:986)
[info]   at effekt.context.ContextOps.in(Context.scala:31)
[info]   at effekt.context.ContextOps.in$(Context.scala:17)
[info]   at effekt.context.Context.effekt$namer$NamerOps$$super$in(Context.scala:41)
[info]   at effekt.namer.NamerOps.in(Namer.scala:979)
[info]   at effekt.namer.NamerOps.in$(Namer.scala:961)
...

I minimized the responsible code to the following:

effect break(): => Int at {}

def main() = {
  try do break()
  with break { resume(42 match {
    case 42 => box { 42 }
    case _ => panic("whaa")
  }) }
  ()
}

Not sure if this is even supposed to work, but we should catch this in earlier phases either way.

[jiribenes]

Works up until v0.55, but not since v0.56, so it's one of these changes: v0.55.0...v0.56.0
If I had to guess, the blame would be on #1219 given where the error comes from.

effekt/effekt/shared/src/main/scala/effekt/core/Transformer.scala

Lines 860 to 862 in 9c3d12e

	def transform(capt: Captures)(using Context): core.Captures = capt match {
	case CaptUnificationVar(role) => Context.panic(pp"$capt should be a concrete capture set in this phase.")
	case CaptureSet(captures) => captures.map(x => x: Symbol) // that is really a noop...

[b-studios]

Annotating box { 42 } at {} seems to fix the issue

[b-studios]

Conjecture: we forget to substitute in inferred coercions (here: Nothing unboxing)