[Bug]: UnitTests fail on arm64 in "Release" mode (gcc-15)

[umlaeute]

Detailed steps on how to reproduce the bug

we are experiencing consistent failures of the UnitTestRunner on Linux/arm64, when building with CONFIG=Release.

-----------------------------------------------------------------
Starting tests in: IntersectingRangedValues / IntersectingRangedValuesTests - iterating over multiple RangedValues...
Completed tests in IntersectingRangedValues / IntersectingRangedValuesTests - iterating over multiple RangedValues
-----------------------------------------------------------------
Starting tests in: SimpleShapedText / getTextRange: LTR Latin text without ligatures - no soft breaks...
Segmentation fault

it appears to work nicely when building with CONFIG=Debug (which i think is the default).

of course, the Release builds are built without -g:

JUCE/extras/UnitTestRunner/Builds/LinuxMakefile/Makefile

Line 67 in 1b460fe

JUCE_CFLAGS += $(JUCE_CPPFLAGS) $(TARGET_ARCH) -O3 $(CFLAGS)

so I've manually changed that line to:

  JUCE_CFLAGS += $(JUCE_CPPFLAGS) $(TARGET_ARCH)  -g -O3 $(CFLAGS)

What is the expected behaviour?

i expect that the unit-tests succeed, regardless of whether i use a Debug or a Release build.

Operating systems

Linux

What versions of the operating systems?

Debian/GNU linux (sid)

the tests are run on a headless build machine without audio hardware.

Architectures

Arm64/aarch64

Stacktrace

Starting tests in: SimpleShapedText / getTextRange: LTR Latin text without ligatures - no soft breaks...

Thread 1 "UnitTestRunner" received signal SIGSEGV, Segmentation fault.
std::_Rb_tree_rebalance_for_erase (__z=0xfffffffffffffffd, __header=...) at ../../../../../src/libstdc++-v3/src/c++98/tree.cc:296
warning: 296    ../../../../../src/libstdc++-v3/src/c++98/tree.cc: No such file or directory
(gdb) bt
#0  std::_Rb_tree_rebalance_for_erase (__z=0xfffffffffffffffd, __header=...) at ../../../../../src/libstdc++-v3/src/c++98/tree.cc:296
#1  0x0000aaaaab1d2a70 in std::__rb_tree::_Node_traits<std::pair<juce::String const, juce::LruCache<juce::String, juce::Array<juce::Unicode::Codepoint, juce::DummyCriticalSection, 0>, 128ul>::Pair>, std::pair<juce::String const, juce::LruCache<juce::String, juce::Array<juce::Unicode::Codepoint, juce::DummyCriticalSection, 0>, 128ul>::Pair>*>::_S_rebalance_for_erase (__z=<optimized out>, __header=...) at /usr/include/c++/15/bits/stl_tree.h:698
#2  std::_Rb_tree<juce::String, std::pair<juce::String const, juce::LruCache<juce::String, juce::Array<juce::Unicode::Codepoint, juce::DummyCriticalSection, 0>, 128ul>::Pair>, std::_Select1st<std::pair<juce::String const, juce::LruCache<juce::String, juce::Array<juce::Unicode::Codepoint, juce::DummyCriticalSection, 0>, 128ul>::Pair> >, std::less<juce::String>, std::allocator<std::pair<juce::String const, juce::LruCache<juce::String, juce::Array<juce::Unicode::Codepoint, juce::DummyCriticalSection, 0>, 128ul>::Pair> > >::_M_erase_aux (this=<optimized out>, __position=...) at /usr/include/c++/15/bits/stl_tree.h:3115
#3  std::_Rb_tree<juce::String, std::pair<juce::String const, juce::LruCache<juce::String, juce::Array<juce::Unicode::Codepoint, juce::DummyCriticalSection, 0>, 128ul>::Pair>, std::_Select1st<std::pair<juce::String const, juce::LruCache<juce::String, juce::Array<juce::Unicode::Codepoint, juce::DummyCriticalSection, 0>, 128ul>::Pair> >, std::less<juce::String>, std::allocator<std::pair<juce::String const, juce::LruCache<juce::String, juce::Array<juce::Unicode::Codepoint, juce::DummyCriticalSection, 0>, 128ul>::Pair> > >::erase[abi:cxx11](std::_Rb_tree_const_iterator<std::pair<juce::String const, juce::LruCache<juce::String, juce::Array<juce::Unicode::Codepoint, juce::DummyCriticalSection, 0>, 128ul>::Pair> >) (this=<optimized out>, __position=...) at /usr/include/c++/15/bits/stl_tree.h:1821
#4  std::map<juce::String, juce::LruCache<juce::String, juce::Array<juce::Unicode::Codepoint, juce::DummyCriticalSection, 0>, 128ul>::Pair, std::less<juce::String>, std::allocator<std::pair<juce::String const, juce::LruCache<juce::String, juce::Array<juce::Unicode::Codepoint, juce::DummyCriticalSection, 0>, 128ul>::Pair> > >::erase (this=<optimized out>, __position=...) at /usr/include/c++/15/bits/stl_map.h:1122
#5  juce::LruCache<juce::String, juce::Array<juce::Unicode::Codepoint, juce::DummyCriticalSection, 0>, 128ul>::get<juce::Array<juce::Unicode::Codepoint, juce::DummyCriticalSection, 0> (&)(juce::String const&)> (this=<optimized out>, 
    key=..., fn=<optimized out>) at ../../../../modules/juce_core/detail/juce_LruCache.h:56
#6  juce::Unicode::performAnalysis (string=...) at ../../../../modules/juce_graphics/unicode/juce_Unicode.cpp:86
#7  0x0000aaaaab132af8 in juce::detail::getLineRanges (data=...) at ../../../../modules/juce_graphics/detail/juce_SimpleShapedText.cpp:1260
#8  juce::detail::SimpleShapedText::shape (this=0xffffffffee40, data=..., options=...) at ../../../../modules/juce_graphics/detail/juce_SimpleShapedText.cpp:1288
#9  0x0000aaaaab1356a4 in juce::detail::SimpleShapedText::SimpleShapedText (this=0xffffffffee40, data=<optimized out>, options=...) at ../../../../modules/juce_graphics/detail/juce_SimpleShapedText.cpp:119
#10 0x0000aaaaab1d608c in juce::detail::SimpleShapedTextTests::runTest (this=this@entry=0xaaaaab9dc5b8 <juce::detail::simpleShapedTextTests>, text=text@entry=0xaaaaab68d5e8 "Some trivial text", maxWidth=maxWidth@entry=100000)
    at ../../../../modules/juce_graphics/detail/juce_SimpleShapedText.cpp:1564
#11 0x0000aaaaab1d6434 in juce::detail::SimpleShapedTextTests::runTest (this=0xaaaaab9dc5b8 <juce::detail::simpleShapedTextTests>) at ../../../../modules/juce_graphics/detail/juce_SimpleShapedText.cpp:1585
#12 0x0000aaaaaaf3b8d8 in juce::UnitTest::performTest (this=0xaaaaab9dc5b8 <juce::detail::simpleShapedTextTests>, newRunner=0xfffffffff128) at ../../../../modules/juce_core/unit_tests/juce_UnitTest.cpp:103
#13 juce::UnitTestRunner::runTests (this=0xfffffffff128, tests=..., randomSeed=<optimized out>) at ../../../../modules/juce_core/unit_tests/juce_UnitTest.cpp:191
#14 0x0000aaaaaab9aca8 in main (argc=<optimized out>, argv=<optimized out>) at ../../Source/Main.cpp:122
(gdb)

Plug-in formats (if applicable)

No response

Plug-in host applications (DAWs) (if applicable)

No response

Testing on the develop branch

I have not tested against the develop branch

Code of Conduct

• I agree to follow the Code of Conduct

[tpoole]

What operating system is this running on?

Our CI runs a release build UnitTestRunner on arm64 Ubuntu and we're not seeing the same problem.

[umlaeute]

as I said: "Debian/GNU linux (sid)"

[tpoole]

Is this on a headless machine? Are we trying to use some fonts that don't exist?

[umlaeute]

Is this on a headless machine?

yes. (as i said: "the tests are run on a headless build machine without audio hardware.")

Are we trying to use some fonts that don't exist?

this might well be, i did not install any specific fonts.

however we are using the same setup of the machines for other architectures (like amd64, armhf, i386, pppc64el and riscv64) and so far this specific problem was only observed for the arm64 arch).
i'm pretty confident that there are no additional packages (like extra fonts) installed on the "working" archs.

you can see the results for of the CI-tests on https://ci.debian.net/packages/j/juce/ (the page currently says 🟢 pass for arm64, because i was cheating and changed the target to Debug for this specific arch; but if you check the details on https://ci.debian.net/packages/j/juce/unstable/arm64/, you will see that only 8.0.10+ds-2 succeeds, and 8.0.10+ds-1 fails - which used to build the UnitTests with Release)

[umlaeute]

you can see the results for of the CI-tests

just for completeness sake: these CI-tests are of course for a JUCE that has Debian specific patches.

however, I've manually run the tests on our amd64 porterbox using a pristine JUCE release, to make sure it is not one of our patches that introduces that regression.
the result was, that pristine JUCE 8.0.10 would crash.

(so my last post was only to say that I do not believe that the arm64 builder does not lack fonts that e.g. the amd64 builder has installed)

[umlaeute]

here's a bit more info.

i've re-run the tests on the develop branch (so far I had tested only on master and the 8.0.10 release), but I'm afraid I still see these crashes.

the only tests that fail for me, are:

• getTextRange: LTR Latin text without ligatures - no soft breaks
• getTextRange: LTR Latin text without ligatures - with soft breaks

if i #if 0 these tests in

JUCE/modules/juce_graphics/detail/juce_SimpleShapedText.cpp

Lines 1574 to 1584 in be58c6d

	beginTest ("getTextRange: LTR Latin text without ligatures - no soft breaks");
	{
	for (auto* testString : testStrings)
	runTest (testString, 100'000.0f);
	}
	beginTest ("getTextRange: LTR Latin text without ligatures - with soft breaks");
	{
	for (auto* testString : testStrings)
	runTest (testString, 60.0f);
	}

, then the UnitTestRunner completes successfully.

I've run a reduced set of the test-suite (-c Text) through valgrind, here are the logs.

---

system information

my (manual) tests are run on a headless (remote) arm64 machine with the following specs:

$ lscpu 
Architecture:                aarch64
  CPU op-mode(s):            32-bit, 64-bit
  Byte Order:                Little Endian
CPU(s):                      8
  On-line CPU(s) list:       0-7
Vendor ID:                   ARM
  Model name:                Neoverse-N1
    Model:                   1
    Thread(s) per core:      1
    Core(s) per socket:      8
    Socket(s):               1
    Stepping:                r3p1
    BogoMIPS:                50.00
    Flags:                   fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp asimdhp cpuid asimdrdm lrcpc dcpop asimddp

$ lsmem 
RANGE                                 SIZE  STATE REMOVABLE BLOCK
0x0000000040000000-0x000000063fffffff  24G online       yes 8-199

Memory block size:                128M
Total online memory:               24G
Total offline memory:               0B

(these specs are most likely different from our CI-runners; but I do not have direct access to the CI-runners)

i guess the software versions are more important, so here they are:

$ g++ --version
g++ (Debian 15.2.0-4) 15.2.0
Copyright (C) 2025 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

the UnitTestRunner directly dylinks against a number of packages (as reported by readelf -d), here's there versions:

package	version
libasound2t64	1.2.14-2
libfontconfig1	2.15.0-2.4
libfreetype6	2.13.3+dfsg-1
libgl1	1.7.0-1+b2
libcurl3t64-gnutls	8.16.0-4
libstdc++6	15.2.0-4
libc6	2.41-12
libgcc-s1	15.2.0-4

full list of dylibs used by the binary (as reported by dd)

package	version
libasound2t64	1.2.14-2
libbrotli1	1.1.0-2+b7
libbz2-1.0	1.0.8-6
libc6	2.41-12
libc6-dev	2.41-12
libcom-err2	1.47.2-3+b3
libcurl3t64-gnutls	8.16.0-4
libexpat1	2.7.3-1
libffi8	3.5.2-2
libfontconfig1	2.15.0-2.4
libfreetype6	2.13.3+dfsg-1
libgcc-s1	15.2.0-4
libgl1	1.7.0-1+b2
libglvnd0	1.7.0-1+b2
libglx0	1.7.0-1+b2
libgmp10	2:6.3.0+dfsg-5
libgnutls30t64	3.8.10-2
libgssapi-krb5-2	1.21.3-5
libhogweed6t64	3.10.2-1
libidn2-0	2.3.8-4
libk5crypto3	1.21.3-5
libkeyutils1	1.6.3-6
libkrb5-3	1.21.3-5
libkrb5support0	1.21.3-5
libldap2	2.6.10+dfsg-1
libnettle8t64	3.10.2-1
libnghttp2-14	1.64.0-1.1+b1
libnghttp3-9	1.12.0-1
libngtcp2-16	1.16.0-1
libngtcp2-crypto-gnutls8	1.16.0-1
libp11-kit0	0.25.9-2
libpng16-16t64	1.6.50-1
libpsl5t64	0.21.2-1.1+b1
librtmp1	2.4+20151223.gitfa8646d.1-3
libsasl2-2	2.1.28+dfsg1-10
libssh2-1t64	1.11.1-1
libssl3t64	3.5.4-1
libstdc++6	15.2.0-4
libtasn1-6	4.20.0-2
libunistring5	1.3-2
libx11-6	2:1.8.12-1
libxau6	1:1.0.11-1
libxcb1	1.17.0-2+b1
libxdmcp6	1:1.1.5-1
libzstd1	1.5.7+dfsg-2
zlib1g	1:1.3.dfsg+really1.3.1-1+b1

full list of installed packages

package	version
adwaita-icon-theme	49.0-1
apt	3.1.8
at-spi2-common	2.58.1-1
autoconf	2.72-3.1
automake	1:1.17-4
autopoint	0.23.1-2
autotools-dev	20240727.1
base-files	14
base-passwd	3.6.7
bash	5.3-1
binutils	2.45-7
binutils-aarch64-linux-gnu	2.45-7
binutils-common	2.45-7
bsdextrautils	2.41.2-4
bubblewrap	0.11.0-2
build-essential	12.12
bzip2	1.0.8-6
ca-certificates	20250419
cmake	4.1.1+really3.31.6-2
cmake-data	4.1.1+really3.31.6-2
comerr-dev	2.1-1.47.2-3+b3
coreutils	9.7-3
cpp	4:15.2.0-4
cpp-15	15.2.0-4
cpp-15-aarch64-linux-gnu	15.2.0-4
cpp-aarch64-linux-gnu	4:15.2.0-4
dash	0.5.12-12
dbus	1.16.2-2
dbus-bin	1.16.2-2
dbus-daemon	1.16.2-2
dbus-session-bus-common	1.16.2-2
dbus-system-bus-common	1.16.2-2
dbus-user-session	1.16.2-2
dconf-gsettings-backend	0.49.0-1
dconf-service	0.49.0-1
debconf	1.5.91
debhelper	13.28
debian-archive-keyring	2025.1
debianutils	5.23.2
devscripts	2.25.19
dh-autoreconf	21
dh-strip-nondeterminism	1.15.0-1
dictionaries-common	1.30.11
diffutils	1:3.10-4
doxygen	1.9.8+ds-2.1
dpkg	1.22.21
dpkg-dev	1.22.21
dwz	0.16-2
emacsen-common	3.0.8
file	1:5.46-5
findutils	4.10.0-3
fontconfig	2.15.0-2.4
fontconfig-config	2.15.0-2.4
fonts-dejavu-core	2.37-8
fonts-dejavu-mono	2.37-8
g++	4:15.2.0-4
g++-15	15.2.0-4
g++-15-aarch64-linux-gnu	15.2.0-4
g++-aarch64-linux-gnu	4:15.2.0-4
gcc	4:15.2.0-4
gcc-15	15.2.0-4
gcc-15-aarch64-linux-gnu	15.2.0-4
gcc-15-base	15.2.0-4
gcc-aarch64-linux-gnu	4:15.2.0-4
gdb	16.3-5
gettext	0.23.1-2+b1
gettext-base	0.23.1-2+b1
gir1.2-atk-1.0	2.58.1-1
gir1.2-atspi-2.0	2.58.1-1
gir1.2-cloudproviders-0.3.0	0.3.6-3
gir1.2-freedesktop	1.86.0-6
gir1.2-freedesktop-dev	1.86.0-6
gir1.2-gdkpixbuf-2.0	2.44.3+dfsg-3
gir1.2-glib-2.0	2.86.0-5
gir1.2-glib-2.0-dev	2.86.0-5
gir1.2-gtk-3.0	3.24.50-2
gir1.2-harfbuzz-0.0	12.1.0-1
gir1.2-javascriptcoregtk-4.1	2.50.1-1
gir1.2-pango-1.0	1.56.3-2
gir1.2-soup-3.0	3.6.5-4
gir1.2-webkit2-4.1	2.50.1-1
girepository-tools	2.86.0-5
glib-networking	2.80.1-1
glib-networking-common	2.80.1-1
glib-networking-services	2.80.1-1
gpg	2.4.8-3
gpg-agent	2.4.8-3
gpgconf	2.4.8-3
gpgv	2.4.8-3
graphviz	2.42.4-3
grep	3.12-1
groff-base	1.23.0-9
gsettings-desktop-schemas	49.1-1
gstreamer1.0-plugins-base	1.26.6-3
gstreamer1.0-plugins-good	1.26.6-1+b1
gtk-update-icon-cache	4.20.1+ds-2
gzip	1.13-1
hicolor-icon-theme	0.18-2
hostname	3.25
hunspell-en-us	1:2020.12.07-4
icu-devtools	76.1-4
init-system-helpers	1.69
intltool-debian	0.35.0+20060710.6
iso-codes	4.18.0-1
krb5-multidev	1.21.3-5
less	668-1
libaa1	1.4p5-51.1+b1
libabsl20240722	20240722.0-4
libacl1	2.3.2-2+b1
libann0	1.1.2+doc-9+b2
libaom3	3.13.1-1
libapparmor1	4.1.0-1
libapt-pkg7.0	3.1.8
libarchive-zip-perl	1.68-1
libarchive13t64	3.7.4-4+b1
libasan8	15.2.0-4
libasound2-data	1.2.14-2
libasound2-dev	1.2.14-2
libasound2t64	1.2.14-2
libaspell15	0.60.8.1-5
libassuan9	3.0.2-2
libasyncns0	0.8-7
libatk-bridge2.0-0t64	2.58.1-1
libatk-bridge2.0-dev	2.58.1-1
libatk1.0-0t64	2.58.1-1
libatk1.0-dev	2.58.1-1
libatomic1	15.2.0-4
libatspi2.0-0t64	2.58.1-1
libatspi2.0-dev	2.58.1-1
libattr1	1:2.5.2-3
libaudit-common	1:4.1.2-1
libaudit1	1:4.1.2-1
libavahi-client3	0.8-17
libavahi-common-data	0.8-17
libavahi-common3	0.8-17
libavc1394-0	0.5.4-5+b2
libavif16	1.3.0-1+b1
libb-hooks-op-check-perl	0.22-3+b2
libbabeltrace1	1.5.11-4+b2
libbinutils	2.45-7
libblkid-dev	2.41.2-4
libblkid1	2.41.2-4
libbrotli-dev	1.1.0-2+b7
libbrotli1	1.1.0-2+b7
libbsd0	0.12.2-2
libbz2-1.0	1.0.8-6
libbz2-dev	1.0.8-6
libc-bin	2.41-12
libc-dev-bin	2.41-12
libc-l10n	2.41-12
libc6	2.41-12
libc6-dbg	2.41-12
libc6-dev	2.41-12
libcaca0	0.99.beta20-6
libcairo-gobject2	1.18.4-1+b1
libcairo-script-interpreter2	1.18.4-1+b1
libcairo2	1.18.4-1+b1
libcairo2-dev	1.18.4-1+b1
libcap-dev	1:2.75-10+b1
libcap-ng0	0.8.5-4+b1
libcap2	1:2.75-10+b1
libcap2-bin	1:2.75-10+b1
libcc1-0	15.2.0-4
libcdparanoia0	3.10.2+debian-14+b1
libcdt5	2.42.4-3
libcgraph6	2.42.4-3
libclang-cpp19	1:19.1.7-7
libclang1-19	1:19.1.7-7
libclass-method-modifiers-perl	2.15-1
libclass-xsaccessor-perl	1.19-4+b5
libclone-perl	0.47-1+b1
libcloudproviders-dev	0.3.6-3
libcloudproviders0	0.3.6-3
libcolord2	1.4.8-1
libcom-err2	1.47.2-3+b3
libcrypt-dev	1:4.4.38-1
libcrypt1	1:4.4.38-1
libctf-nobfd0	2.45-7
libctf0	2.45-7
libcups2t64	2.4.14-1
libcurl3t64-gnutls	8.16.0-4
libcurl4-gnutls-dev	8.16.0-4
libcurl4t64	8.16.0-4
libdatrie-dev	0.2.13-4
libdatrie1	0.2.13-4
libdav1d7	1.5.1-1
libdb5.3t64	5.3.28+dfsg2-10
libdbus-1-3	1.16.2-2
libdbus-1-dev	1.16.2-2
libdconf1	0.49.0-1
libde265-0	1.0.16-1
libdebconfclient0	0.280
libdebhelper-perl	13.28
libdebuginfod-common	0.193-3
libdebuginfod1t64	0.193-3
libdeflate-dev	1.23-2
libdeflate0	1.23-2
libdevel-callchecker-perl	0.009-2
libdpkg-perl	1.22.21
libdrm-amdgpu1	2.4.125-2
libdrm-common	2.4.125-2
libdrm2	2.4.125-2
libduktape207	2.7.0-2+b2
libdv4t64	1.0.0-17.1+b1
libdw1t64	0.193-3
libdynaloader-functions-perl	0.004-2
libedit2	3.1-20250104-1
libegl-dev	1.7.0-1+b2
libegl-mesa0	25.2.4-1
libegl1	1.7.0-1+b2
libegl1-mesa-dev	25.2.4-1
libelf1t64	0.193-3
libenchant-2-2	2.8.2+dfsg1-3
libencode-locale-perl	1.05-3
libepoxy-dev	1.5.10-2
libepoxy0	1.5.10-2
libevdev2	1.13.4+dfsg-1
libevent-2.1-7t64	2.1.12-stable-10+b1
libexpat1	2.7.3-1
libexpat1-dev	2.7.3-1
libffi-dev	3.5.2-2
libffi8	3.5.2-2
libfile-dirlist-perl	0.05-3
libfile-homedir-perl	1.006-2
libfile-listing-perl	6.16-1
libfile-stripnondeterminism-perl	1.15.0-1
libfile-touch-perl	0.12-2
libfile-which-perl	1.27-2
libflac-dev	1.5.0+ds-2
libflac14	1.5.0+ds-2
libflite1	2.2-7
libfmt10	10.1.1+ds1-4
libfontconfig-dev	2.15.0-2.4
libfontconfig1	2.15.0-2.4
libfreetype-dev	2.13.3+dfsg-1
libfreetype6	2.13.3+dfsg-1
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[tpoole]

Thank you.

We've also narrowed the problem down a bit. We can reproduce the issue using GCC 15.2 (GCC 14 works fine), and it happens when using the Makefile generated by the Projucer, but not when configuring the build using CMake. Hopefully a diff of the compilation flags will give us a bit more information.

[reuk]

I think this is caused by a bug in GCC 15.2, although I haven't isolated the problem yet.

If I change modules/juce_core/detail/juce_LruCache.h line 77 from

while (list.size() >= maxEntries)

to

while (map.size() >= maxEntries)

then the crash no longer manifests.

I can also get the crash to disappear by swapping the order of the map and list data members in the LruCache.

If I adjust the develop version of LruCache::get() so that the end of the function looks like this:

const auto mapIteratorPair = map.emplace (std::move (key), Pair { std::move (value), {} });

jassert (mapIteratorPair.second);

mapIteratorPair.first->second.listIterator = list.insert (list.end(), mapIteratorPair.first);
std::cout << "list size " << list.size() << '\n';

if (list.size() == 0)
{
    std::cout << "aborting\n";
    std::terminate();
}

return mapIteratorPair.first->second.value;

Then I see this output:

list size 0
aborting
terminate called without an active exception

So it appears that, immediately after a call to list.insert(), the size of the list is still 0. According to the C++ standard, this should only happen if the insertion fails and an exception is thrown, but no exception is thrown in this case.

I tried setting a watchpoint on the list size member (list._M_impl._M_node._M_size) and this doesn't appear to be updated during the insert call. However, given that the same STL code works at lower optimisation levels and with different compilers, I think the bug is more likely to be in the compiler than in libstdc++.

Based on all of that, I think the most likely explanation is that GCC 15.2 is applying a broken optimisation.

I know that compiler bugs are fairly rare, so please let me know if you have any evidence that might point to a bug in JUCE instead. In the meantime, I'd suggest switching to a different compiler until this issue is resolved. We can push a workaround for this particular issue, but it still seems risky to use GCC 15.2 if there's a chance that it might emit similarly broken code elsewhere (e.g. in a location not covered by a unit test).

[umlaeute]

Based on all of that, I think the most likely explanation is that GCC 15.2 is applying a broken optimisation.

in this case it would be great if you could report a bug with GCC.

[reuk]

We've pushed a workaround for this bug here:
7aa7f49

Unfortunately the unit test runner is a big program, so if we were to submit a bug report given the information we know currently it wouldn't be particularly actionable, and I don't think we can justify the time investment to reduce the program down to a minimal test-case.

[umlaeute]

thanks for the fix (although the link you posted seems to be broken).

whether the time investment is worth it, is something you must decide.
(afaik, gcc is still the most common compiler for Linux distributions, and I would expect distributions to quickly adopt the latest and greatest release, making the fallout of such a bug considerable... but then: I'm not going to investigate this myself any further either ¯⁠\⁠_⁠༼⁠ᴼ⁠ل͜⁠ᴼ⁠༽⁠_⁠/⁠¯)