This bug was originally reported in Fedora, but I was asked to report it upstream by Fedora maintainer.

Description of problem:

Fedora 40 build of libsigc++20 is missing the sigc::internal::signal_impl::unreference_exec() symbol. This causes some third-party proprietary applications (like Horizon Client) to segfault because they refer to and expect that symbol to be present.

Version-Release number of selected component (if applicable):

libsigc++20-2.12.1-2.fc40.x86_64

How reproducible:

Always

Steps to Reproduce:

1. Download Debian build from https://packages.debian.org/sid/amd64/libsigc++-2.0-0v5/download
2. Extract the libsigc-2.0.so.0.0.0 shared library
3. dnf install libsigc++20
4. abidiff libsigc-2.0.so.0.0.0 /usr/lib64/libsigc-2.0.so.0.0.0 |c++filt

Actual results:

Functions changes summary: 0 Removed, 0 Changed, 0 Added function
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable
Function symbols changes summary: 1 Removed, 0 Added function symbol not referenced by debug info
Variable symbols changes summary: 0 Removed, 0 Added variable symbol not referenced by debug info

1 Removed function symbol not referenced by debug info:

  [D] sigc::internal::signal_impl::unreference_exec()

Expected results:

No ABI difference.

Additional info:

This affects all Fedora releases, including rawhide. Ubuntu package (https://packages.ubuntu.com/noble/libsigc++-2.0-0v5) has the same issue. RHEL9 package libsigc++20-2.10.7-2.el9 has the same issue. Oddly, RHEL8's 2.10.0-6.el8 does have this symbol and a bunch of others, too.

I suspect compiler and/or linker issue. The compiler versions are as follows:

There seem to be some differences in how Debian and Fedora build libsigc++20. Debian has this in their rules file:

export DEB_BUILD_MAINT_OPTIONS = hardening=+all
export DEB_LDFLAGS_MAINT_APPEND = -Wl,-O1 -Wl,-z,defs

%:
	dh $@ --buildsystem=meson --without autoreconf

override_dh_auto_configure:
	dh_auto_configure -- \
		-Dmaintainer-mode=true \
		-Dwarnings=max \
		-Dbuild-documentation=true

while Fedora simply does this:

%build
%meson -Dbuild-documentation=true
%meson_build