More aggressive GCC 12 -Wmaybe-uninitialized when using <functional>

[Stephan Bergmann <sbergman@redhat.com>]

Compared to GCC 11 (at least gcc-c++-11.1.1-3.fc34.x86_64), recent GCC 
12 trunk emits two "unhelpful" -Wmaybe-uninitialized for

> $ cat test.cc
> #include <functional>
> using fn = std::function<void()>;
> fn f(fn x) {
>     fn a;
>     a = x;
>     return x;
> }

> $ ~/gcc/trunk/inst/bin/g++ -c -Wmaybe-uninitialized -O2 test.cc
> In file included from ~/gcc/trunk/inst/include/c++/12.0.0/bits/stl_function.h:60,
>                  from ~/gcc/trunk/inst/include/c++/12.0.0/functional:49,
>                  from test.cc:1:
> In function ‘std::_Require<std::__not_<std::__is_tuple_like<_Tp> >, std::is_move_constructible<_Tp>, std::is_move_assignable<_Tp> > std::swap(_Tp&, _Tp&) [with _Tp = void (*)(const std::_Any_data&)]’,
>     inlined from ‘void std::function<_Res(_ArgTypes ...)>::swap(std::function<_Res(_ArgTypes ...)>&) [with _Res = void; _ArgTypes = {}]’ at ~/gcc/trunk/inst/include/c++/12.0.0/bits/std_function.h:529:11,
>     inlined from ‘std::function<_Res(_ArgTypes ...)>& std::function<_Res(_ArgTypes ...)>::operator=(const std::function<_Res(_ArgTypes ...)>&) [with _Res = void; _ArgTypes = {}]’ at ~/gcc/trunk/inst/include/c++/12.0.0/bits/std_function.h:442:20,
>     inlined from ‘fn f(fn)’ at test.cc:5:9:
> ~/gcc/trunk/inst/include/c++/12.0.0/bits/move.h:204:11: warning: ‘<unnamed>.std::function<void()>::_M_invoker’ may be used uninitialized [-Wmaybe-uninitialized]
>   204 |       _Tp __tmp = _GLIBCXX_MOVE(__a);
>       |           ^~~~~
> In file included from ~/gcc/trunk/inst/include/c++/12.0.0/functional:59,
>                  from test.cc:1:
> ~/gcc/trunk/inst/include/c++/12.0.0/bits/std_function.h: In function ‘fn f(fn)’:
> ~/gcc/trunk/inst/include/c++/12.0.0/bits/std_function.h:442:9: note: ‘<anonymous>’ declared here
>   442 |         function(__x).swap(*this);
>       |         ^~~~~~~~~~~~~
> In file included from ~/gcc/trunk/inst/include/c++/12.0.0/bits/stl_function.h:60,
>                  from ~/gcc/trunk/inst/include/c++/12.0.0/functional:49,
>                  from test.cc:1:
> In function ‘std::_Require<std::__not_<std::__is_tuple_like<_Tp> >, std::is_move_constructible<_Tp>, std::is_move_assignable<_Tp> > std::swap(_Tp&, _Tp&) [with _Tp = std::_Any_data]’,
>     inlined from ‘void std::function<_Res(_ArgTypes ...)>::swap(std::function<_Res(_ArgTypes ...)>&) [with _Res = void; _ArgTypes = {}]’ at ~/gcc/trunk/inst/include/c++/12.0.0/bits/std_function.h:527:11,
>     inlined from ‘std::function<_Res(_ArgTypes ...)>& std::function<_Res(_ArgTypes ...)>::operator=(const std::function<_Res(_ArgTypes ...)>&) [with _Res = void; _ArgTypes = {}]’ at ~/gcc/trunk/inst/include/c++/12.0.0/bits/std_function.h:442:20,
>     inlined from ‘fn f(fn)’ at test.cc:5:9:
> ~/gcc/trunk/inst/include/c++/12.0.0/bits/move.h:204:11: warning: ‘*(std::_Any_data*)((char*)&<unnamed> + offsetof(std::function, std::function<void()>::<unnamed>))’ may be used uninitialized [-Wmaybe-uninitialized]
>   204 |       _Tp __tmp = _GLIBCXX_MOVE(__a);
>       |           ^~~~~
> In file included from ~/gcc/trunk/inst/include/c++/12.0.0/functional:59,
>                  from test.cc:1:
> ~/gcc/trunk/inst/include/c++/12.0.0/bits/std_function.h: In function ‘fn f(fn)’:
> ~/gcc/trunk/inst/include/c++/12.0.0/bits/std_function.h:442:9: note: ‘<anonymous>’ declared here
>   442 |         function(__x).swap(*this);
>       |         ^~~~~~~~~~~~~

This appears to be an issue with more aggressive -Wmaybe-uninitialized 
in GCC 12 vs. 11, rather than an issue with changes to <functional> in 
libstdc++ 12 vs. 11, as effectively the same warnings are emitted when I 
use GCC 12 with libstdc++ 11 with

> $ ~/gcc/trunk/inst/bin/g++ -c -Wmaybe-uninitialized -O2 -nostdinc++ -isystem /usr/include/c++/11 -isystem /usr/include/c++/11/x86_64-redhat-linux test.cc

The warnings may technically be correct, and I'm not sure whether this 
is something that should be addressed in the GCC code emitting the 
warnings or in the libstdc++ <functional> implementation.

(I found this when building LibreOffice with recent GCC 12 trunk.)