[Bug c++/105050] New: error: expression '' is not a constant expression

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105050

            Bug ID: 105050
           Summary: error: expression '<statement>' is not a constant
                    expression
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

template<typename _Tp>
  struct expected
  {
    void _M_swap_val(expected&) { }

    void _M_swap_unex(expected&) { }

    void _M_swap_val_unex(expected&) { }

    bool _M_has_value = true;

    constexpr void
    swap(expected& __x)
    {
      if (this->has_value())
      {
        if (__x.has_value())
          this->_M_swap_val(__x);
        else
          this->_M_swap_val_unex(__x);
      }
      else
      {
        if (__x.has_value())
          __x._M_swap_val_unex(*this);
        else
          this->_M_swap_unex(__x);
      }
    }

    constexpr bool has_value() const noexcept { return this->_M_has_value; }
  };

constexpr bool
test_swap()
{
  expected<int> e1, e2;
  e1.swap(e2);

  return true;
}

static_assert(test_swap());



This doesn't give a very helpful diagnostic:

x.ii:43:24: error: non-constant condition for static assertion
   43 | static_assert(test_swap());
      |               ~~~~~~~~~^~
x.ii:43:24:   in 'constexpr' expansion of 'test_swap()'
x.ii:38:10: error: 'constexpr void expected<_Tp>::swap(expected<_Tp>&) [with
_Tp = int]' called in a constant expression
   38 |   e1.swap(e2);
      |   ~~~~~~~^~~~
x.ii:13:5: note: 'constexpr void expected<_Tp>::swap(expected<_Tp>&) [with _Tp
= int]' is not usable as a 'constexpr' function because:
   13 |     swap(expected& __x)
      |     ^~~~
x.ii:15:7: error: expression '<statement>' is not a constant expression
   15 |       if (this->has_value())
      |       ^~


The problem is that the _M_swap_val, _M_swap_unex and _M_swap_val_unex
functions are not constexpr, but the diagnostic doesn't make that obvious.