Match-error in JVM backend when emitting HK-type with type lambda contianing primtive

[WojciechMazur]

Based on the OpenCB failure in dfianthdl/dfhdl

Most likely caused inside referenceTypeSignature() in the JVM backend while handling Ljava/lang/Object;LCheck<Z>; signature (the <Z> part might be invalid here, we probably should not have a priimitive type in generic

scala3/compiler/src/dotty/tools/backend/jvm/GenericSignatureVisitor.scala

Lines 84 to 122 in a584906

	@tailrec private def referenceTypeSignature(): Unit = getCurrentAndSkip() match {
	case 'L' =>
	val start = index
	var seenDollar = false
	while (!isClassNameEnd(current)) {
	seenDollar ||= current == '$'
	index += 1
	}
	// OPT: avoid allocations when collecting only nested classes and only a top-level class is encountered
	val topLevelIndex = index
	lazy val names = {
	val n = new java.lang.StringBuilder(32)
	n.append(sig, start, topLevelIndex)
	n
	}
	if ((current == '.' || seenDollar) || !nestedOnly) {
	visitInternalName(names.toString)
	}
	typeArguments()
	while (current == '.') {
	skip()
	names.append('$')
	appendUntil(names, isClassNameEnd)
	visitInternalName(names.toString)
	typeArguments()
	}
	accept(';')
	case 'T' =>
	skipUntilDelimiter(';')
	skip()
	case '[' =>
	if (isBaseType(current)) skip()
	else referenceTypeSignature()
	}

Compiler version

3.9.0-RC5 (backported)

Last good release: 3.10.0-RC1-bin-20260803-11ca8f7-NIGHTLY
First bad release: 3.10.0-RC1-bin-20260804-9db016f-NIGHTLY

Biect points to #26632 - it's a root cause, the underlying issue was already there but hidden

Minimized code

import compiletime.ops.int.>

trait Check[Cond[T <: Int] <: Boolean]:
  def apply(arg: Int): Unit

type CondT = [t <: Int] =>> t > 0

val c: Check[CondT] = new Check[CondT]:
  def apply(arg: Int): Unit = ()

Output

Error while emitting test$package$
Z (of class java.lang.Character)
Error while emitting test$package
Z (of class java.lang.Character)
Error while emitting test$package$$anon$1
Z (of class java.lang.Character)
3 errors found
Compilation failed

Expectation

Should compile

[SolalPirelli]

Yup, this fails -Xverify-signatures so we're emitting nonsense and then trying to read it. On it.