AssertionError: no TypeBounds allowed, on wildcard (?) in extends clause

[TomasMikula]

Compiler version

3.6.3

Worked in 3.5.2, so this is a regression.

Minimized code

sealed trait Foo[T]
class Bar extends Foo[?]

def mkFoo[T]: Foo[T] =
  ???

def test: Unit =
  mkFoo match
    case _ => ()

Output (click arrow to expand)

% ~/Downloads/scala3-3.6.3-aarch64-apple-darwin/bin/scalac test.scala               

  unhandled exception while running MegaPhase{protectedAccessors, extmethods, uncacheGivenAliases, checkStatic, elimByName, hoistSuperArgs, forwardDepChecks, specializeApplyMethods, tryCatchPatterns, patternMatcher} on test.scala

  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 -Xno-enrich-error-messages.


     while compiling: test.scala
        during phase: MegaPhase{protectedAccessors, extmethods, uncacheGivenAliases, checkStatic, elimByName, hoistSuperArgs, forwardDepChecks, specializeApplyMethods, tryCatchPatterns, patternMatcher}
                mode: Mode(ImplicitsEnabled)
     library version: version 2.13.15
    compiler version: version 3.6.3
            settings: 

Exception in thread "main" java.lang.AssertionError: no TypeBounds allowed
	at dotty.tools.dotc.core.TypeApplications$.noBounds(TypeApplications.scala:22)
	at dotty.tools.dotc.core.TypeApplications$.argTypes$extension$$anonfun$1(TypeApplications.scala:557)
	at scala.collection.immutable.List.mapConserve(List.scala:473)
	at dotty.tools.dotc.core.TypeApplications$.argTypes$extension(TypeApplications.scala:557)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.getTypeArgs$1(Space.scala:677)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.getChildren$1$$anonfun$1(Space.scala:689)
	at scala.collection.immutable.List.flatMap(List.scala:294)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.getChildren$1(Space.scala:682)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.rec$1(Space.scala:695)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.decompose(Space.scala:715)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$Parts$.unapply(Space.scala:733)
	at dotty.tools.dotc.transform.patmat.Typ.decompose(Space.scala:101)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.decompose(Space.scala:118)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.computeSimplify(Space.scala:134)
	at dotty.tools.dotc.transform.patmat.Space.simplify(Space.scala:70)
	at dotty.tools.dotc.transform.patmat.Space.simplify$(Space.scala:52)
	at dotty.tools.dotc.transform.patmat.Typ.simplify(Space.scala:92)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.simplify(Space.scala:115)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.$anonfun$3(Space.scala:129)
	at scala.collection.immutable.List.map(List.scala:247)
	at scala.collection.immutable.List.map(List.scala:79)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.computeSimplify(Space.scala:129)
	at dotty.tools.dotc.transform.patmat.Space.simplify(Space.scala:70)
	at dotty.tools.dotc.transform.patmat.Space.simplify$(Space.scala:52)
	at dotty.tools.dotc.transform.patmat.Or.simplify(Space.scala:112)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.simplify(Space.scala:115)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.$anonfun$3(Space.scala:129)
	at scala.collection.immutable.List.map(List.scala:247)
	at scala.collection.immutable.List.map(List.scala:79)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.computeSimplify(Space.scala:129)
	at dotty.tools.dotc.transform.patmat.Space.simplify(Space.scala:70)
	at dotty.tools.dotc.transform.patmat.Space.simplify$(Space.scala:52)
	at dotty.tools.dotc.transform.patmat.Or.simplify(Space.scala:112)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.simplify(Space.scala:115)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.recur$1(Space.scala:943)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.checkReachability(Space.scala:971)
	at dotty.tools.dotc.transform.patmat.SpaceEngine$.checkMatch(Space.scala:976)
	at dotty.tools.dotc.transform.PatternMatcher.transformMatch(PatternMatcher.scala:52)
	at dotty.tools.dotc.transform.MegaPhase.goMatch(MegaPhase.scala:816)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:383)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:454)
	at dotty.tools.dotc.transform.MegaPhase.mapDefDef$1(MegaPhase.scala:265)
	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:268)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:452)
	at dotty.tools.dotc.transform.MegaPhase.loop$1(MegaPhase.scala:465)
	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:465)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:376)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:454)
	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:272)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:452)
	at dotty.tools.dotc.transform.MegaPhase.loop$1(MegaPhase.scala:465)
	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:465)
	at dotty.tools.dotc.transform.MegaPhase.mapPackage$1(MegaPhase.scala:396)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:399)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:454)
	at dotty.tools.dotc.transform.MegaPhase.transformUnit(MegaPhase.scala:481)
	at dotty.tools.dotc.transform.MegaPhase.run(MegaPhase.scala:493)
	at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:380)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:334)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:373)
	at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:343)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
	at dotty.tools.dotc.Run.runPhases$1(Run.scala:336)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:383)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:395)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:69)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:395)
	at dotty.tools.dotc.Run.compileSources(Run.scala:282)
	at dotty.tools.dotc.Run.compile(Run.scala:267)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:37)
	at dotty.tools.dotc.Driver.process(Driver.scala:201)
	at dotty.tools.dotc.Driver.process(Driver.scala:169)
	at dotty.tools.dotc.Driver.process(Driver.scala:181)
	at dotty.tools.dotc.Driver.main(Driver.scala:211)
	at dotty.tools.MainGenericCompiler$.run$1(MainGenericCompiler.scala:162)
	at dotty.tools.MainGenericCompiler$.main(MainGenericCompiler.scala:186)
	at dotty.tools.MainGenericCompiler.main(MainGenericCompiler.scala)

[Gedochao]

Last good stable version: 3.6.2
(no good nightlies in between)
First bad nightly: 3.6.3-RC1-bin-20241107-caac72a-NIGHTLY
First bad stable version: 3.6.3
Still broken at 3.7.0-RC1-bin-20250210-7ac31d0-NIGHTLY

Seems to point at caac72a
cc @jchyb

Edit: Updated bisect in a comment below: #22518 (comment)

[jchyb]

I can try to look at this but caac72a is unrelated (it was a change to stdlib method only accessible in macros, no macros here though)

[Gedochao]

I can try to look at this but caac72a is unrelated (it was a change to stdlib method only accessible in macros, no macros here though)

@jchyb Yeah... I'm looking at it right now, and I think my initial bisect may have been wrong.
Something seems off about this... the issue actually appears in earlier nightlies, as far as 3.6.1, but it's not present in stable 3.6.1 and 3.6.2 releases.
Let me investigate some more first, sorry for the confusion. 😕

[jchyb]

No worries, I thought there might have been some mix up between the issues

[Gedochao]

Okay... updated bisect:

Last good stable version: 3.6.2
Last good nightly version: 3.6.1-RC1-bin-20241018-9d78307-NIGHTLY
First bad nightly version: 3.6.2-RC1-bin-20241020-6e32627-NIGHTLY
First bad stable version: 3.6.3
Still broken at 3.7.0-RC1-bin-20250210-7ac31d0-NIGHTLY

The error seems to have been introduced somewhere in 9d78307..6e32627, so one of:

• 34f1c54
• bcbdb5e
• 40956e1
• 654bc6c
• 022b1ff
• ad09ab8
• 6e32627

The bisect script points at 34f1c54, but that can't be right, either... 😕

6e32627 seems to me to be the most likely culprit. cc @dwijnand
It's also tagged for 3.6.3-RC1, which would explain stable 3.6.2 being unaffected.