[Bug c++/105050] error: expression '<statement>' is not a constant expression

["ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Created attachment 52687
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52687&action=edit
elaborate non-constexpr if stmt branches

Seems we issued this error because we determined that neither branch of the
first 'if' is a valid constexpr body (since each branch will end up calling a
non-constexpr function), and so the 'if' is a non-constant "expression"
regardless of control flow, which makes 'swap' non-constexpr overall.

The attached diff makes us elaborate why neither branch of an 'if' is
non-constant in this case, similar to how we diagnose disjunctions during
satisfaction.  For this testcase we'd now get:

105050.C:43:24: error: non-constant condition for static assertion
   43 | static_assert(test_swap());
      |               ~~~~~~~~~^~
105050.C:43:24:   in ‘constexpr’ expansion of ‘test_swap()’
105050.C:38:10: error: ‘constexpr void expected<_Tp>::swap(expected<_Tp>&)
[with _Tp = int]’ called in a constant expression
   38 |   e1.swap(e2);
      |   ~~~~~~~^~~~
105050.C: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)
      |     ^~~~
105050.C:15:7: note: neither branch of ‘if’ is a valid ‘constexpr’ body
because:
   15 |       if (this->has_value())
      |       ^~
105050.C:17:9: note: neither branch of ‘if’ is a valid ‘constexpr’ body
because:
   17 |         if (__x.has_value())
      |         ^~
105050.C:18:28: error: call to non-‘constexpr’ function ‘void
expected<_Tp>::_M_swap_val(expected<_Tp>&) [with _Tp = int]’
   18 |           this->_M_swap_val(__x);
      |           ~~~~~~~~~~~~~~~~~^~~~~
105050.C:4:10: note: ‘void expected<_Tp>::_M_swap_val(expected<_Tp>&) [with _Tp
= int]’ declared here
    4 |     void _M_swap_val(expected&) { }
      |          ^~~~~~~~~~~
105050.C:20:33: error: call to non-‘constexpr’ function ‘void
expected<_Tp>::_M_swap_val_unex(expected<_Tp>&) [with _Tp = int]’
   20 |           this->_M_swap_val_unex(__x);
      |           ~~~~~~~~~~~~~~~~~~~~~~^~~~~
105050.C:8:10: note: ‘void expected<_Tp>::_M_swap_val_unex(expected<_Tp>&)
[with _Tp = int]’ declared here
    8 |     void _M_swap_val_unex(expected&) { }
      |          ^~~~~~~~~~~~~~~~
105050.C:24:9: note: neither branch of ‘if’ is a valid ‘constexpr’ body
because:
   24 |         if (__x.has_value())
      |         ^~
105050.C:25:31: error: call to non-‘constexpr’ function ‘void
expected<_Tp>::_M_swap_val_unex(expected<_Tp>&) [with _Tp = int]’
   25 |           __x._M_swap_val_unex(*this);
      |           ~~~~~~~~~~~~~~~~~~~~^~~~~~~
105050.C:8:10: note: ‘void expected<_Tp>::_M_swap_val_unex(expected<_Tp>&)
[with _Tp = int]’ declared here
    8 |     void _M_swap_val_unex(expected&) { }
      |          ^~~~~~~~~~~~~~~~
105050.C:27:29: error: call to non-‘constexpr’ function ‘void
expected<_Tp>::_M_swap_unex(expected<_Tp>&) [with _Tp = int]’
   27 |           this->_M_swap_unex(__x);
      |           ~~~~~~~~~~~~~~~~~~^~~~~
105050.C:6:10: note: ‘void expected<_Tp>::_M_swap_unex(expected<_Tp>&) [with
_Tp = int]’ declared here
    6 |     void _M_swap_unex(expected&) { }
      |          ^~~~~~~~~~~~

We definitely should clarify the initial error, but I'm not sure if we want the
recursive elaboration, might be too noisy..