unwindset parsing: Work around spurious GCC 12 warning

[tautschnig]

GCC 12 wrongly complains about the use of an uninitialized variable. Work around by doing a poor man's version of a variable with an accompanying Boolean.

The build failure was:

In file included from /usr/include/c++/12/bits/stl_algobase.h:64,
                 from /usr/include/c++/12/bits/stl_tree.h:63,
                 from /usr/include/c++/12/map:60,
                 from ../util/symbol_table_base.h:9,
                 from ../util/symbol_table.h:9,
                 from ../goto-programs/goto_model.h:15,
                 from unwindset.h:15,
                 from unwindset.cpp:9:
In constructor ‘constexpr std::pair<_T1, _T2>::pair(_U1&&, _U2&&) [with _U1 = std::__cxx11::basic_string<char>&; _U2 = unsigned int&; typename std::enable_if<(std::_PCC<true, _T1, _T2>::_MoveConstructiblePair<_U1, _U2>() && std::_PCC<true, _T1, _T2>::_ImplicitlyMoveConvertiblePair<_U1, _U2>()), bool>::type <anonymous> = true; _T1 = dstringt; _T2 = unsigned int]’,
    inlined from ‘void unwindsett::parse_unwindset_one_loop(std::string, message_handlert&)’ at unwindset.cpp:174:28:
/usr/include/c++/12/bits/stl_pair.h:535:42: error: ‘*(unsigned int*)((char*)&thread_nr + offsetof(nonstd::optional_lite::optionalt<unsigned int>,nonstd::optional_lite::optional<unsigned int>::contained))’ may be used uninitialized [-Werror=maybe-uninitialized]
  535 |         : first(std::forward<_U1>(__x)), second(std::forward<_U2>(__y)) { }
      |                                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
unwindset.cpp: In member function ‘void unwindsett::parse_unwindset_one_loop(std::string, message_handlert&)’:
unwindset.cpp:39:23: note: ‘*(unsigned int*)((char*)&thread_nr + offsetof(nonstd::optional_lite::optionalt<unsigned int>,nonstd::optional_lite::optional<unsigned int>::contained))’ was declared here
   35 |   optionalt<unsigned> thread_nr;
      |                       ^~~~~~~~~

when using GCC 12: g++ (GCC) 12.2.1 20221121 (Red Hat 12.2.1-4)

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[martin-cs]

Frustrating but understandable.

[NlightNFotis]

Would it be better if instead of complicating the code to suppress the warning, we just suppressed the warning using a pragma for that file only, like this:

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-result"
    write(foo, bar, baz);
#pragma GCC diagnostic pop

?

https://stackoverflow.com/a/26003732

[thomasspriggs]

I would much prefer suppressing the warning rather than working around it by removing usage of optionalt. The compilers analysis is clearly incorrect in this case. I also get a variation of this error from other at least one other part of the code base when I try to build locally. This suppressing the warning be done across the build as a quick work-around by adding the following to cmake - -DCMAKE_CXX_FLAGS="-Wno-maybe-uninitialized"

[codecov]

Codecov Report

Base: 78.49% // Head: 78.49% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (f4dfdd1) compared to base (5678ed6).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7540   +/-   ##
========================================
  Coverage    78.49%   78.49%           
========================================
  Files         1667     1667           
  Lines       191437   191440    +3     
========================================
+ Hits        150261   150264    +3     
  Misses       41176    41176           

Impacted Files	Coverage Δ
src/goto-instrument/unwindset.cpp	85.84% <100.00%> (+0.41%)	⬆️

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[thomasspriggs]

Alternative approach here - #7542

[tautschnig]

Closing in favour of #7542.