<compare>: std::_Literal_zero has an impact on real code. #4359
Description
Describe the bug
Our type std::_Literal_zero has an impact on real code.
From the comment: #4332 (comment)
Command-line test case
#include <compare>
template<typename T, typename U>
constexpr bool not_equal(T lhs, U rhs)
requires(requires(T lhs, U rhs) { lhs != rhs; })
{
return lhs != rhs;
}
template<typename T, typename U>
constexpr bool not_equal(T lhs, U rhs)
requires(!requires(T lhs, U rhs) { lhs != rhs; })
{
return false;
}
int main() {
constexpr int int_zero = 0;
constexpr unsigned int unsigned_zero = 0;
constexpr auto ordering = 1 <=> 1;
//constexpr bool b = not_equal(ordering, unsigned_zero);
constexpr bool b = not_equal(ordering, int_zero);
return b == false;
}
C:\Temp>cl /EHsc /W4 /WX /std:c++latest .\repro.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.39.33321 for x64
Copyright (C) Microsoft Corporation. All rights reserved.
/std:c++latest is provided as a preview of language features from the latest C++
working draft, and we're eager to hear about bugs and suggestions for improvements.
However, note that these features are provided as-is without support, and subject
to changes or removal as the working draft evolves. See
https://go.microsoft.com/fwlink/?linkid=2045807 for details.
repro.cpp
.\repro.cpp(7): error C7595: 'std::_Literal_zero::_Literal_zero': call to immediate function is not a constant expression
.\repro.cpp(7): note: failure was caused by a read of a variable outside its lifetime
.\repro.cpp(7): note: see usage of 'rhs'
.\repro.cpp(7): note: the template instantiation context (the oldest one first) is
.\repro.cpp(22): note: see reference to function template instantiation 'bool not_equal<std::strong_ordering,int>(T,U)' being compiled
with
[
T=std::strong_ordering,
U=int
]
Expected behavior
Should compile
STL version
Additional context
Other compiler libraries compile the code: https://godbolt.org/z/1vbYbq5b7
We added std::_Literal_zero because of this issue: #3581
Previously using _Literal_zero = decltype(nullptr); warned this code:
#include <compare>
auto b = 1 <=> 2 < 0;
int main() {}clang-cl /std:c++20 -Wzero-as-null-pointer-constant main.cpp
main.cpp(3,20): warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
auto b = 1 <=> 2 < 0;
^
nullptr
1 warning generated.