| Issue |
141563
|
| Summary |
consteval and constexpr static variable do not work well together
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
fekir
|
Consider following example
----
#include <iostream>
#include <string_view>
#define USECONSTEVAL
#ifdef USECONSTEVAL
#define CEXPR consteval
#else
#define CEXPR constexpr
#endif
CEXPR std::string_view name(){
return __PRETTY_FUNCTION__;
}
CEXPR std::string_view name2(){
constexpr auto function = std::string_view(__PRETTY_FUNCTION__);
constexpr static auto value = [] <std::size_t...Idxs>(std::string_view str, std::index_sequence<Idxs...>)
{
return std::array{str[Idxs]..., '\0'};
}(function, std::make_index_sequence<function.size()>{});
static_assert(!std::string_view(value).empty());
return std::string_view(value);
}
template <std::size_t...Idxs>
constexpr auto substring_as_array(std::string_view str, std::index_sequence<Idxs...>)
{
return std::array{str[Idxs]..., '\0'};
}
CEXPR std::string_view name3(){
constexpr auto function = std::string_view(__PRETTY_FUNCTION__);
constexpr static auto value = substring_as_array(function, std::make_index_sequence<function.size()>{});
static_assert(!std::string_view(value).empty());
return std::string_view(value);
}
struct helper {
static constexpr std::string_view value = name();
};
CEXPR std::string_view name4(){
constexpr auto value = helper::value;
return std::string_view{value.data(), value.size()};
}
int main(){
static_assert(not name().empty());
std::cout << "__PRETTY_FUNCTION__: \"" << name() << "\"\n";
static_assert(not name2().empty());
std::cout << "__PRETTY_FUNCTION__: \"" << name2() << "\"\n";
static_assert(not name3().empty());
std::cout << "__PRETTY_FUNCTION__: \"" << name3() << "\"\n";
static_assert(not name4().empty());
std::cout << "__PRETTY_FUNCTION__: \"" << name4() << "\"\n";
}
----
by toggling the macro `USECONSTEVAL`, the output of this sample program changes from
----
__PRETTY_FUNCTION__: "std::string_view name()"
__PRETTY_FUNCTION__: "std::string_view name2()"
__PRETTY_FUNCTION__: "std::string_view name3()"
__PRETTY_FUNCTION__: "std::string_view name()"
----
to
----
__PRETTY_FUNCTION__: "std::string_view name()"
__PRETTY_FUNCTION__: ""
__PRETTY_FUNCTION__: ""
__PRETTY_FUNCTION__: "std::string_view name()"
----
To the best of my knowledge, there is no reason why `consteval` instead of `constexpr` should change the meaning of the functions `name2` and `name3`.
Confusingly, none of the `static_assert` that check for emptiness fail, even when the return value is in fact empty.
In case it is relevant, GCC is not affected by this issue, the behavior between `constexpr` and `consteval` is consistent
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