[Bug c++/77943] New: Optimization incorrectly commons noexcept calls with non-noexcept calls

["redbeard0531 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77943

            Bug ID: 77943
           Summary: Optimization incorrectly commons noexcept calls with
                    non-noexcept calls
           Product: gcc
           Version: 6.2.1
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redbeard0531 at gmail dot com
  Target Milestone: ---

When compiled with -O0 and -01 the program behaves correctly and only calls
std::terminate() when passed an argument. With -O2 and -O3 it doesn't call
std::terminate() which means that it allowed an exception to escape from a
noexcept function in violation of the standard. 

> g++ -O0 noexcept_test.cpp && ./a.out ; ./a.out 1
should die: 0
should die: 1
terminate called after throwing an instance of 'int'
Aborted (core dumped)

> g++ -O1 noexcept_test.cpp && ./a.out ; ./a.out 1
should die: 0
should die: 1
terminate called after throwing an instance of 'int'
Aborted (core dumped)

> g++ -O2 noexcept_test.cpp && ./a.out ; ./a.out 1
should die: 0
should die: 1

> g++ -O3 noexcept_test.cpp && ./a.out ; ./a.out 1
should die: 0
should die: 1

The actual behavior with -O2 and -O3 depends on which way the if block is
written. The attached version is the scariest, but if you swap the if and else
blocks and remove the ! from the condition, the executable will always call
std::terminate() even when it shouldn't.