Prospective destructors in C++ - c++

Question

I have this code and this outputs the following:
link to the following example
https://godbolt.org/z/z8Pn9GsTv
template <typename T>
struct A1 {
A1() {
std::cout << "construction of a1" << std::endl;
}
~A1() {
std::cout << "destruction of a1" << std::endl;
}
~A1() requires (std::is_same_v<T,int>) {
std::cout << "it is an int" << std::endl;
}
};
int main() {
A1 <int>a;
return 0;
}
output:
construction of a1
destruction of a1
but swapping places of destructors it gives other result:
link to the code
https://godbolt.org/z/vxj7dPqaj
template <typename T>
struct A1 {
A1() {
std::cout << "construction of a1" << std::endl;
}
~A1() requires (std::is_same_v<T,int>) {
std::cout << "it is an int" << std::endl;
}
~A1() {
std::cout << "destruction of a1" << std::endl;
}
};
output:
construction of a1
it is an int
wondering is this a bug?

Answer 1

That's indeed a reported Clang bug1, as noted by Quimby.
Note that the second snippet (the one with the the constrained destructor first) doesn't really "work" in Clang, which just ignores the second destructor2.
Also note that, unlike gcc, at the moment I'm writing, Clang doesn't seem to have implemented [P0848R3] (which is about conditional trivial special member functions) yet3.
1) https://bugs.llvm.org/show_bug.cgi?id=50570
2) See e.g.: https://godbolt.org/z/rff7qfK65
3) See the reported values of the feature test macro __cpp_concepts, e.g. here: https://godbolt.org/z/P4z3Pj5vT

Answer 2

In your 2'nd code,
Try changing this:
A1 <int>a;
To this:
A1 <double>a;
And you'll get the output:
construction of a1
destruction of a1 // With MSVC & GCC compiler
My interpretation here is that when the condition for the first constructor (with requires) fails, the 2'nd constructor is called which prints "destruction of a1" and destroys a.
Here is a more detailed explanation...

Answer 3

On your class template, you have two destructors definition recipe. During instantiation the The compiler gets the "first" recipe that match the signature needed. Depending the compiler depending the result. I think clang, gcc and mingw will provide the same result, msvc approach is different. Better pattern for partial specialization disambiguation precedence chain?