** Description changed:
Summary
=======
Installing libgcc-16-dev alongside clang-N is enough to break clang++: it
fails to link any C++ program with `ld: cannot find -lstdc++`. This happens
even though the system default gcc is 15 and libstdc++-15-dev is available.
libgcc-16-dev is routinely pulled in transitively (e.g. by libboost-all-dev via
the fortran chain), so users hit this without ever asking for gcc-16.
Steps to reproduce (clean ubuntu:26.04)
=======================================
- apt-get update
- apt-get install -y --no-install-recommends clang-22 libgcc-16-dev
- echo 'int main(){}' > /t.cpp
- clang++-22 /t.cpp -o /t
+ apt-get update
+ apt-get install -y --no-install-recommends clang-22 libgcc-16-dev
+ echo 'int main(){}' > /t.cpp
+ clang++-22 /t.cpp -o /t
+
+ Alternate reproduction (Dockerfile)
+ ===================================
+ The following Dockerfile reproduces the failure self-contained;
+ `docker build` fails on the final layer (that failure IS the reproduction):
+
+ FROM ubuntu:26.04
+ RUN apt-get update \
+ && DEBIAN_FRONTEND=noninteractive apt-get install -y
--no-install-recommends \
+ clang-22 libgcc-16-dev \
+ && rm -rf /var/lib/apt/lists/*
+ RUN echo 'int main() { return 0; }' > /t.cpp \
+ && clang++-22 /t.cpp -o /t
Expected
========
Linking succeeds (default gcc is 15 and libstdc++-15-dev provides libstdc++).
Actual
======
- clang++-22: warning: future releases of the clang compiler will prefer
GCC installations containing libstdc++ include directories;
'/usr/lib/gcc/x86_64-linux-gnu/15' would be chosen over
'/usr/lib/gcc/x86_64-linux-gnu/16' [-Wgcc-install-dir-libstdcxx]
- /usr/bin/x86_64-linux-gnu-ld.bfd: cannot find -lstdc++: No such file or
directory
- clang++-22: error: linker command failed with exit code 1
+ clang++-22: warning: future releases of the clang compiler will prefer GCC
installations containing libstdc++ include directories;
'/usr/lib/gcc/x86_64-linux-gnu/15' would be chosen over
'/usr/lib/gcc/x86_64-linux-gnu/16' [-Wgcc-install-dir-libstdcxx]
+ /usr/bin/x86_64-linux-gnu-ld.bfd: cannot find -lstdc++: No such file or
directory
+ clang++-22: error: linker command failed with exit code 1
Root cause
==========
libgcc-16-dev populates /usr/lib/gcc/x86_64-linux-gnu/16/ with the GCC
runtime objects (crtbegin.o, libgcc.a). clang's GCC-installation detector
selects the highest-versioned directory containing those objects (16 over 15)
and assumes libstdc++ lives in the same prefix. Since libstdc++-16-dev is not
installed, -lstdc++ cannot be found. gcc-16-base on its own does NOT trigger
this (it creates no runtime dir); libgcc-16-dev is the minimal trigger.
The clang heuristic itself is being addressed upstream in LLVM
(https://github.com/llvm/llvm-project/pull/145056 - "Take libstdc++ into
account during GCC detection"; the -Wgcc-install-dir-libstdcxx warning
above comes from that work, and a future release will prefer the
complete dir, i.e. 15). This report is about the Ubuntu packaging
experience: a default clang install silently lands in a non-linkable
state via ordinary transitive deps.
Suggested fixes (packaging side)
================================
- Ship a clang driver default that pins --gcc-install-dir to the distro
default gcc, or
- Have the clang package Depend/Recommend a matching libstdc++ for the
highest installable gcc runtime, or
- Document the libstdc++-<N>-dev requirement prominently.
Environment
===========
- Ubuntu 26.04
- clang-22 (llvm-toolchain-22), default gcc 15.2.0, gcc-16-base 16-20260322
- Reproduced in a clean ubuntu:26.04 container.
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https://bugs.launchpad.net/bugs/2154746
Title:
Installing libgcc-16-dev and clang-N together breaks clang++ (cannot
find -lstdc++)
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