solver3: Explicitly declare and define ~Solver

On recent armfhf and s390x builds in the PPA, the compiler seems to have
generated the destructor at the wrong point where the definitions where
not yet complete, and it does seem ill-advised to rely on a
default-constructed destructor in the solver for future ABI sake.

In file included from /usr/include/c++/14/vector:66,
                 from /<<PKGBUILDDIR>>/obj-arm-linux-gnueabihf/include/apt-pkg/cachefilter.h:14:
/usr/include/c++/14/bits/stl_vector.h: In instantiation of ‘constexpr std::_Vector_base<_Tp, _Alloc>::~_Vector_base() [with _Tp = APT::Solver::Work; _Alloc = std::allocator<APT::Solver::Work>]’:
/usr/include/c++/14/bits/stl_vector.h:531:7:   required from here
  531 |       vector() = default;
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:369:49: error: invalid use of incomplete type ‘struct APT::Solver::Work’
  369 |                       _M_impl._M_end_of_storage - _M_impl._M_start);
      |                       ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
In file included from /<<PKGBUILDDIR>>/apt-pkg/edsp.cc:22:
/<<PKGBUILDDIR>>/obj-arm-linux-gnueabihf/include/apt-pkg/solver3.h:87:11: note: forward declaration of ‘struct APT::Solver::Work’
   87 |    struct Work;
      |           ^~~~
/usr/include/c++/14/bits/stl_vector.h: In instantiation of ‘constexpr std::_Vector_base<_Tp, _Alloc>::~_Vector_base() [with _Tp = APT::Solver::Solved; _Alloc = std::allocator<APT::Solver::Solved>]’:
/usr/include/c++/14/bits/stl_vector.h:531:7:   required from here
  531 |       vector() = default;
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:369:49: error: invalid use of incomplete type ‘struct APT::Solver::Solved’
  369 |                       _M_impl._M_end_of_storage - _M_impl._M_start);
      |                       ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
/<<PKGBUILDDIR>>/obj-arm-linux-gnueabihf/include/apt-pkg/solver3.h:88:11: note: forward declaration of ‘struct APT::Solver::Solved’
   88 |    struct Solved;
      |           ^~~~~~

2 files changed, 3 insertions, 0 deletions

diff --git a/apt-pkg/solver3.cc b/apt-pkg/solver3.cc
index c53911a80..052142658 100644
--- a/apt-pkg/solver3.cc
+++ b/apt-pkg/solver3.cc
@@ -216,6 +216,8 @@ APT::Solver::Solver(pkgCache &cache, pkgDepCache::Policy &policy, EDSP::Request:
    rootState->decision = Decision::MUST;
 }
 
+APT::Solver::~Solver() = default;
+
 // This function determines if a work item is less important than another.
 bool APT::Solver::Work::operator<(APT::Solver::Work const &b) const
 {
diff --git a/apt-pkg/solver3.h b/apt-pkg/solver3.h
index a333d684a..2331af869 100644
--- a/apt-pkg/solver3.h
+++ b/apt-pkg/solver3.h
@@ -287,6 +287,7 @@ class Solver
 
    // \brief Basic solver initializer. This cannot fail.
    Solver(pkgCache &Cache, pkgDepCache::Policy &Policy, EDSP::Request::Flags requestFlags);
+   ~Solver();
 
    // Assume that the variable is decided as specified.
    [[nodiscard]] bool Assume(Var var, bool decision, const Clause *reason = nullptr);