[Bug c++/91705] New: operator++ broken in constexpr floating point code

[john at johnmaddock dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91705

            Bug ID: 91705
           Summary: operator++ broken in constexpr floating point code
           Product: gcc
           Version: 9.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: john at johnmaddock dot co.uk
  Target Milestone: ---

The following short program was accepted by gcc-8, but results in an error with
gcc-9.1:

template <class T>
constexpr T test_increment(T t)
{
  return ++t;
}

int main()
{
  constexpr double t = test_increment(2.0f);
  static_assert(t == 3);
}

With:

/home/john/t.cpp: In function ‘int main()’:
/home/john/t.cpp:15:38:   in ‘constexpr’ expansion of
‘test_increment<float>(2.0e+0f)’
/home/john/t.cpp:15:43: error: ‘((float)1 + 2.0e+0f)’ is not a constant
expression
   15 |   constexpr double t = test_increment(2.0f);
      |                                           ^
/home/john/t.cpp:16:19: error: non-constant condition for static assertion
   16 |   static_assert(t == 3);

The problem is present for all floating point types (float, double, long double
and __float128), but not integers.  Replacing ++t with t+1 fixes the problem.