[Clang] Placement-new in constant evaluation can give indeterminate result #117294
Description
The following program shouldn't be accepted per [expr.const]/5.18.2.1 as corrected by CWG2922 (since the constructed array type is unsuitable).
However, Clang currently accepts it and give different results in different compilation. Godbolt link.
#include <new>
#include <typeinfo>
constexpr int N = []
{
struct S {
int a[1];
};
S s;
::new (s.a) int[1][2][3][4]();
return s.a[0];
}();
template<int X>
struct tag {};
void unknown(const std::type_info&) noexcept;
int main()
{
unknown(typeid(tag<N>));
}Moreover, the following seems valid because the array type is suitable, but there are still indeterminate results. Godbolt link.
(The correct N is 0.)
#include <new>
#include <typeinfo>
constexpr int N = []
{
struct S {
int a[1];
};
S s;
::new (s.a) int[1](); // s.a is converted to the poiner to s.a[0]
return s.a[0];
}();
template<int X>
struct tag {};
void unknown(const std::type_info&) noexcept;
int main()
{
unknown(typeid(tag<N>));
}This can be also trigged by the current resolution of LWG3436 (so labeled c++20).