Here is what I just commited.

I try to use the asm trick in the _GLIBCXX_DEBUG_VERIFY_COND_AT but 
didn't notice any enhancement. So for now I kept my solution to just 
have a non-constexpr call compiler error.

I fix my patch to use __builtin_is_constant_evaluated rather than 
std::is_constant_evaluated in __valid_range.

     * include/bits/stl_algobase.h (__memmove): Return _Tp*.
     (__memmove): Loop as long as __n is not 0.
     (__copy_move<>::__copy_m): Likewise.
     (__copy_move_backward<>::__copy_move_b): Likewise.
     * testsuite/25_algorithms/copy/constexpr.cc: Add check on copied 
values.
     * testsuite/25_algorithms/copy_backward/constexpr.cc: Likewise.
     * testsuite/25_algorithms/copy/constexpr_neg.cc: New.
     * testsuite/25_algorithms/copy_backward/constexpr.cc: New.

     * include/debug/forward_list
(_Sequence_traits<__debug::forward_list<>>::_S_size): Returns __dp_sign
     distance when not empty.
     * include/debug/list
     (_Sequence_traits<__debug::list<>>::_S_size): Likewise.
     * include/debug/helper_functions.h (__dp_sign_max_size): New
     _Distance_precision enum entry.
     * include/debug/safe_iterator.h
     (__copy_move_a(_II, _II, const _Safe_iterator<>&)): Check for output
     iterator _M_can_advance as soon as input range distance precision is
     strictly higher than __dp_size.
     (__copy_move_a(const _Safe_iterator<>&, const _Safe_iterator<>&,
     const _Safe_iterator<>&)): Likewise.
     (__copy_move_backward_a(_II, _II, const _Safe_iterator<>&)): Likewise.
     (__copy_move_backward_a(const _Safe_iterator<>&,
     const _Safe_iterator<>&, const _Safe_iterator<>&)): Likewise.
     (__equal_aux(_II, _II, const _Safe_iterator<>&)): Likewise.
     (__equal_aux(const _Safe_iterator<>&,
     const _Safe_iterator<>&, const _Safe_iterator<>&)): Likewise.

François

On 9/27/19 6:45 PM, Jonathan Wakely wrote:
> On 27/09/19 18:24 +0200, François Dumont wrote:
>> On 9/27/19 2:11 PM, Jonathan Wakely wrote:
>>> On 19/09/19 22:27 +0200, François Dumont wrote:
>>>> Hi
>>>>
>>>>     I start working on making recently added constexpr tests to 
>>>> work in Debug mode.
>>>
>>> The attached patch seems to be necessary for that, right?
>>>
>>>
>> On my side I had done this, almost the same.
>>
>> For the moment there is a FIXME in macros.h to find out how to 
>> generate a nice compilation error when the condition is not meant.
>>
>> static_assert can't be called in this context, too bad.
>>
>> I also try to define a function with a 
>> __attribute__((__error__("because"))) attribute. But when I make it 
>> constexpr gcc complains about missing definition. When I provide a 
>> definition gcc complains that this attribute must be on a 
>> declaration. And when I split declaration and definition gcc does not 
>> produce the expected compilation error.
>
> Yes, I've tried similar things without success.
>
>> Unless you have the solution I consider that we need help from the 
>> front-end.
>>
>> For the moment if Debug mode finds a problem it will be reported as 
>> _M_error function not being constexpr !
>
> A reasonable workaround is to do:
>
> #ifdef _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
>  if (__builtin_is_constant_evaluated())
>    asm("Debug Mode assertion failed");
>  else
> #endif
>  if (!(Cond))
>    __gnu_debug::_Error_formatter::...
>
> The builtin is available even for C++98, whereas
> std::is_constant_evaluated() is only available for C++20.
>
> This produces errors that include lines like:
>
> asm.cc:12:17:   in ‘constexpr’ expansion of ‘f(-1)’
> asm.cc:4:7: error: inline assembly is not a constant expression
>    4 |       asm("debug mode assertion failed");
>      |       ^~~
> asm.cc:8:3: note: in expansion of macro ‘CHECK’
>    8 |   _GLIBCXX_ASSERT(i > 0);
>      |   ^~~~~
> asm.cc:4:7: note: only unevaluated inline assembly is allowed in a 
> ‘constexpr’ function in C++2a
>    4 |       asm("debug mode assertion failed");
>      |       ^~~
> asm.cc:8:3: note: in expansion of macro ‘CHECK’
>    8 |   CHECK(i > 0);
>      |   ^~~~~
>
> It's not ideal, but it does show the failed condition and the text
> "debug mode assertion failed" (or whatever message you choose to use
> there).
>
>
>