apt: pkg-config-test fails with many compiler errors

[Nick Rosbrook]

Log: https://autopkgtest.ubuntu.com/results/autopkgtest-resolute/resolute/arm64/a/apt/20260108_153333_170ae@/log.gz

Relevant part:

76s autopkgtest [15:08:51]: test pkg-config-test: [-----------------------
 77s Using include dir: /usr/include
 77s build g++ -std=c++23: In file included from pkgconfigtest.cc:3:
 77s /usr/include/apt-pkg/cacheiterators.h:33:2: warning: #warning apt-pkg/cacheiterators.h should not be included directly, include apt-pkg/pkgcache.h instead [-Wcpp]
 77s 33 | #warning apt-pkg/cacheiterators.h should not be included directly, include apt-pkg/pkgcache.h instead
 77s | ^~~~~~~
 77s /usr/include/apt-pkg/cacheiterators.h:47:55: error: ‘pkgCache’ has not been declared
 77s 47 | template<typename Str, typename Itr> class APT_PUBLIC pkgCache::Iterator {
 77s | ^~~~~~~~
 77s /usr/include/apt-pkg/cacheiterators.h:47:74: error: expected unqualified-id before ‘{’ token
 77s 47 | template<typename Str, typename Itr> class APT_PUBLIC pkgCache::Iterator {
 77s | ^
 77s /usr/include/apt-pkg/cacheiterators.h:106:18: error: ‘pkgCache’ has not been declared
 77s 106 | class APT_PUBLIC pkgCache::GrpIterator: public Iterator<Group, GrpIterator> {
 77s | ^~~~~~~~
 77s /usr/include/apt-pkg/cacheiterators.h:106:39: error: expected unqualified-id before ‘:’ token
 77s 106 | class APT_PUBLIC pkgCache::GrpIterator: public Iterator<Group, GrpIterator> {
 77s | ^
 77s /usr/include/apt-pkg/cacheiterators.h:147:18: error: ‘pkgCache’ has not been declared
 77s 147 | class APT_PUBLIC pkgCache::PkgIterator: public Iterator<Package, PkgIterator> {
 77s | ^~~~~~~~
 77s /usr/include/apt-pkg/cacheiterators.h:147:39: error: expected unqualified-id before ‘:’ token
 77s 147 | class APT_PUBLIC pkgCache::PkgIterator: public Iterator<Package, PkgIterator> {
 77s | ^
 77s /usr/include/apt-pkg/cacheiterators.h:193:18: error: ‘pkgCache’ has not been declared
 77s 193 | class APT_PUBLIC pkgCache::SrcVerIterator : public Iterator<SourceVersion, SrcVerIterator>
 77s | ^~~~~~~~
 77s /usr/include/apt-pkg/cacheiterators.h:193:43: error: expected unqualified-id before ‘:’ token
 77s 193 | class APT_PUBLIC pkgCache::SrcVerIterator : public Iterator<SourceVersion, SrcVerIterator>
 77s | ^
 77s /usr/include/apt-pkg/cacheiterators.h:218:18: error: ‘pkgCache’ has not been declared
 77s 218 | class APT_PUBLIC pkgCache::VerIterator : public Iterator<Version, VerIterator> {
 77s | ^~~~~~~~
 77s /usr/include/apt-pkg/cacheiterators.h:218:40: error: expected unqualified-id before ‘:’ token
 77s 218 | class APT_PUBLIC pkgCache::VerIterator : public Iterator<Version, VerIterator> {
 77s | ^
 77s /usr/include/apt-pkg/cacheiterators.h:311:18: error: ‘pkgCache’ has not been declared
 77s 311 | class APT_PUBLIC pkgCache::DescIterator : public Iterator<Description, DescIterator> {
 77s | ^~~~~~~~

[... SNIP ... ]

 83s /usr/include/c++/15/bits/forward_list.tcc:55:36: required from ‘std::_Fwd_list_base<_Tp, _Allocator>::_Base_ptr std::_Fwd_list_base<_Tp, _Allocator>::_M_insert_after(const_iterator, _Args&& ...) [with _Args = {const pkgCache::VerIterator&}; _Tp = pkgCache::VerIterator; _Alloc = std::allocator<pkgCache::VerIterator>; _Base_ptr = std::_Fwd_list_node_base*; const_iterator = std::_Fwd_list_base<pkgCache::VerIterator, std::allocator<pkgCache::VerIterator> >::const_iterator]’
 83s 55 | _Node_ptr __thing = _M_create_node(std::forward<_Args>(__args)...);
 83s | ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 83s /usr/include/c++/15/bits/forward_list.h:1339:30: required from ‘void std::forward_list<_Tp, _Allocator>::push_front(const _Tp&) [with _Tp = pkgCache::VerIterator; _Alloc = std::allocator<pkgCache::VerIterator>]’
 83s 1339 | { this->_M_insert_after(cbefore_begin(), __val); }
 83s | ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
 83s /usr/include/apt-pkg/cacheset.h:1005:18: required from here
 83s 1005 | _cont.push_front(V);
 83s | ~~~~~~~~~~~~~~~~^~~
 83s /usr/include/c++/15/bits/new_allocator.h:191:11: error: invalid use of incomplete type ‘class pkgCache::VerIterator’
 83s 191 | { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
 83s | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 83s /usr/include/apt-pkg/pkgcache.h:148:10: note: forward declaration of ‘class pkgCache::VerIterator’
 83s 148 | class VerIterator;
 83s | ^~~~~~~~~~~
 83s autopkgtest [15:08:58]: test pkg-config-test: -----------------------]
 84s autopkgtest [15:08:59]: test pkg-config-test: - - - - - - - - - - results - - - - - - - - - -
 84s pkg-config-test FAIL non-zero exit status 4