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).
