Crash: `val _ = <MethodHandle>` causes cyclic reference

[fwbrasil]

Compiler version

3.9.0-RC4. Reproduces on JDK 21 and 25.

Minimized code

//> using scala 3.9.0-RC4

import java.lang.invoke.MethodHandle

object M:
    def h: MethodHandle = ???
    val _ = h

Output (click arrow to expand)

Details

exception caught when loading class ClassSpecializer: Cyclic reference involving class ClassSpecializer
exception caught when loading class SpeciesData: Cyclic reference involving class ClassSpecializer

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

  An unhandled exception was thrown in the compiler.

     while compiling: Repro.scala
        during phase: MegaPhase{protectedAccessors, extmethods, uncacheGivenAliases, checkStatic, elimByName, hoistSuperArgs, forwardDepChecks, specializeApplyMethods, tryCatchPatterns, patternMatcher}
                mode: Mode(ImplicitsEnabled)
     library version: version 3.9.0-RC4
    compiler version: version 3.9.0-RC4

Exception in thread "main" dotty.tools.dotc.core.CyclicReference: Cyclic reference involving class ClassSpecializer
Compilation failed

[fwbrasil]

I was able to work it around by not using val _ so it's not critical for Kyo.

[SethTisue]

perhaps the same as #18392? I chose to close that one and keep this one open, since this one has a minimal reproducer

I wonder what's special about ClassSpecializer

[Gedochao]

Last good release: 3.9.0-RC1-bin-20260506-0de5613-NIGHTLY
First bad release: 3.9.0-RC1-bin-20260507-b8210ea-NIGHTLY
The first bad commit is 4204700 (#25788)
cc @lalala194 @SolalPirelli @Florian3k

Reproduces with JDK 19+, works fine on JDK 17 & 18.