error: interface dependency /usr/lib64/lib{gtk-4}@/usr/bin/g++/ is unknown

[wroyca]

Interface dependencies act up when attempting to use libadwaita in a C++ package. No amount of tweaking resolves the situation, whether switching from c to cc or trying various methods to export (cc.export.libs, c.export.libs, etc.).

error: interface dependency /usr/lib64/liba{gtk4}@/usr/bin/g++/ is unknown
  info: mentioned in *.export.libs of target ../foo-gcc/libadwaita-1.5.0+4/libadwaita/libs{adwaita}
  info: is it a prerequisite of ../foo-gcc/libadwaita-1.5.0+4/libadwaita/libs{adwaita}?
  info: while applying rule cxx.compile to update ../foo-gcc/libfoo/libfoo/objs{foo}
  info: while applying rule cxx.link to update ../foo-gcc/libfoo/libfoo/libs{foo}
  info: while applying rule cxx.link to update ../foo-gcc/foo/foo/exe{foo}
  info: while applying rule build.alias to update ../foo-gcc/foo/dir{foo/}
  info: while applying rule build.alias to update ../foo-gcc/dir{foo/}
error: interface dependency /usr/lib64/libs{gtk4}@/usr/bin/g++/ is unknown
  info: mentioned in *.export.libs of target ../foo-gcc/libadwaita-1.5.0+4/libadwaita/liba{adwaita}
  info: is it a prerequisite of ../foo-gcc/libadwaita-1.5.0+4/libadwaita/liba{adwaita}?
  info: while applying rule cxx.compile to update ../foo-gcc/libfoo/libfoo/obja{foo}
  info: while applying rule cxx.link to update ../foo-gcc/libfoo/libfoo/liba{foo}
  info: while applying rule bin.lib to update ../foo-gcc/libfoo/libfoo/lib{foo}
  info: while applying rule build.alias to update ../foo-gcc/libfoo/dir{libfoo/}
  info: while applying rule build.alias to update ../foo-gcc/dir{libfoo/}

Attempting a minimal C library linked in a C++ project setup works, so I suspect there's something else I'm missing, or perhaps there's something unusual in the GTK pkg-config file. For now, the workaround involves forcing config.c to use g++:

#!/usr/bin/env bash

if ! cat /etc/os-release | grep -q "Fedora"; then
  echo "Warning: Your distribution is not Fedora, which may result in outdated GTK."
fi

set -x

bdep new -t empty foo
bdep new -t empty bar
bdep new -t empty baz

cd foo

bdep new --package -l c++ -t exe foo
bdep new --package -l c++ -t lib libfoo

cd ../bar

bdep new --package -l c -t lib libbar

cd ../baz

bdep new --package -l c -t lib libbaz

cd ..

# Configure dependencies repositories

cat <<EOF > foo/repositories.manifest
: 1
summary: foo project repository

:
role: prerequisite
location: ../bar.git#main

:
role: prerequisite
location: https://github.com/build2-packaging/libadwaita.git#main

#:
#role: prerequisite
#location: ../libadwaita.git#main
EOF

cat <<EOF > bar/repositories.manifest
: 1
summary: bar project repository

:
role: prerequisite
location: ../baz.git#main
EOF

# Update dependency declarations in project manifests.

sed -i 's/#depends: libhello \^1\.0\.0/depends: libfoo == $/'                 foo/foo/manifest
sed -i 's/#depends: libhello \^1\.0\.0/depends: libbar\ndepends: libadwaita/' foo/libfoo/manifest
sed -i 's/#depends: libhello \^1\.0\.0/depends: libbaz/'                      bar/libbar/manifest

# Update dependency declarations in packages buildfile

sed -i 's/#import libs += libhello%lib{hello}/import libs += libfoo%lib{foo}/'                                   foo/foo/foo/buildfile
sed -i 's/#import xxxx_libs += libhello%lib{hello}/import intf_libs += libbar%lib{bar} libadwaita%lib{adwaita}/' foo/libfoo/libfoo/buildfile
sed -i 's/#import xxxx_libs += libhello%lib{hello}/import intf_libs += libbaz%lib{baz}/'                         bar/libbar/libbar/buildfile

# Disable tests in bar.

rm -rf bar/libbar/tests

# Override bar source code to export "hello"

cat <<EOF > bar/libbar/libbar/bar.c
#include <libbar/bar.h>

int
hello ()
{
  return 0;
}
EOF

cat <<EOF > bar/libbar/libbar/bar.h
#pragma once

#include <libbar/export.h>

#ifdef __cplusplus
extern "C"
{
#endif

LIBBAR_SYMEXPORT int
hello ();

#ifdef __cplusplus
}
#endif
EOF

# Override foo source code to import "hello" and "adw_init" from bar and libadwaita, respectively.

cat <<EOF > foo/libfoo/libfoo/foo.cxx
#include <libfoo/foo.hxx>
#include <libbar/bar.h>
#include <adwaita.h>

#include <ostream>
#include <stdexcept>

using namespace std;

namespace foo
{
  void say_hello (ostream& o, const string& n)
  {
    hello ();

    adw_init();

    if (n.empty ())
      throw invalid_argument ("empty name");

    o << "Hello, " << n << '!' << endl;
  }
}
EOF

# Commit above sed operations.

cd foo
git add .
git commit -m "Initial commit"

cd ../bar
git add .
git commit -m "Initial commit"

cd ../baz
git add .
git commit -m "Initial commit"

# Now we'll attempt to build foo along with its dependencies.
#

cd ../foo

c='\e[1;33m'
r='\e[1;0m'

set +x

echo -e "$(
cat << EOM
${c}Initial attempt.${r}
EOM
)"

set -x

bdep init -C @gcc cc
bdep update

set +x

echo -e "$(
cat << EOM
${c}Second attempt. Force config.c to `g++`${r}
EOM
)"

set -x

rm -rf .bdep/
bdep init -C @gcc cc config.c=g++ --wipe
bdep update

set +x

Also related: https://cpplang.slack.com/archives/CDJ0Z991S/p1720210865895529?thread_ts=1710423736.413539&cid=CDJ0Z991S