Found while working on JDK-8369699.

Reproduces on my AVX512 machine, but also with AVX2, not with AVX1:
java -XX:CompileCommand=compileonly,Test::test* -XX:CompileCommand=printcompilation,Test::* -Xbatch Test.java

#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/home/empeter/Documents/oracle/jdk-fork3/open/src/hotspot/share/opto/vector.cpp:369), pid=2372263, tid=2372278
# assert(vbox->is_CheckCastPP()) failed: should be expanded
#
# JRE version: Java(TM) SE Runtime Environment (27.0) (fastdebug build 27-internal-empeter.open)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 27-internal-empeter.open, mixed mode, tiered, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x1c33e08] PhaseVector::expand_vbox_node_helper(Node*, Node*, TypeInstPtr const*, TypeVect const*, VectorSet&)+0xf8
#
# Core dump will be written. Default location: Determined by the following: "/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h %d %F" (alternatively, falling back to /home/empeter/Documents/oracle/jdk-fork3/build/linux-x64-debug/jdk/bin/core.2372263)
#
# If you would like to submit a bug report, please visit:
# https://bugreport.java.com/bugreport/crash.jsp
#

--------------- S U M M A R Y ------------

Command Line: -XX:CompileCommand=compileonly,Test::test* -XX:CompileCommand=printcompilation,Test::* -Xbatch --add-modules=ALL-DEFAULT jdk.compiler/com.sun.tools.javac.launcher.SourceLauncher Test.java

Host: emanuel.peter, 11th Gen Intel(R) Core(TM) i7-11850H @ 2.50GHz, 16 cores, 29G, Oracle Linux Server release 9.7
Time: Mon Jan 12 11:19:25 2026 CET elapsed time: 3.362626 seconds (0d 0h 0m 3s)

--------------- T H R E A D ---------------

Current thread (0x00007f06ac1d6d70): JavaThread "C2 CompilerThread0" daemon [_thread_in_native, id=2372278, stack(0x00007f0690a56000,0x00007f0690b56000) (1024K)]


Current CompileTask:
C2:3362 152 b 4 Test::test2 (39 bytes)

Stack: [0x00007f0690a56000,0x00007f0690b56000], sp=0x00007f0690b50f40, free space=1003k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x1c33e08] PhaseVector::expand_vbox_node_helper(Node*, Node*, TypeInstPtr const*, TypeVect const*, VectorSet&)+0xf8 (vector.cpp:369)
V [libjvm.so+0x1c342c9] PhaseVector::expand_vbox_node(VectorBoxNode*)+0x89 (vector.cpp:303)
V [libjvm.so+0x1c345eb] PhaseVector::expand_vbox_nodes()+0x16b (vector.cpp:112)
V [libjvm.so+0x1c3615d] PhaseVector::optimize_vector_boxes()+0x10d (vector.cpp:56)
V [libjvm.so+0xbb2f4b] Compile::Optimize()+0x1c3b (compile.cpp:2383)
V [libjvm.so+0xbb5173] Compile::Compile(ciEnv*, ciMethod*, int, Options, DirectiveSet*)+0x2023 (compile.cpp:862)
V [libjvm.so+0x9cc3e8] C2Compiler::compile_method(ciEnv*, ciMethod*, int, bool, DirectiveSet*)+0x498 (c2compiler.cpp:147)
V [libjvm.so+0xbc4660] CompileBroker::invoke_compiler_on_method(CompileTask*)+0x780 (compileBroker.cpp:2345)
V [libjvm.so+0xbc5ec0] CompileBroker::compiler_thread_loop()+0x530 (compileBroker.cpp:1989)
V [libjvm.so+0x112635b] JavaThread::thread_main_inner()+0x13b (javaThread.cpp:776)
V [libjvm.so+0x1bb26f6] Thread::call_run()+0xb6 (thread.cpp:242)
V [libjvm.so+0x18082d8] thread_native_entry(Thread*)+0x118 (os_linux.cpp:860)