clang++ accept illegal code

[llvmbot]

Bugzilla Link	37886
Resolution	DUPLICATE
Resolved on	Jun 21, 2018 11:25
Version	unspecified
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@dwblaikie,@DougGregor,@zygoloid

Extended Description

The code is as follow:

#include

template
struct SimpleAllocator
{
template friend struct SimpleAllocator;

using value_type = T;
using pointer = T *;

struct rebind { template <typename U> using other = SimpleAllocator<U>; };

using propagate_on_container_copy_assignment = std::true_type;
using propagate_on_container_move_assignment = std::true_type;
using propagate_on_container_swap = std::true_type;

explicit SimpleAllocator(void * p) : state(p) {}

template <typename U>
SimpleAllocator(SimpleAllocator<U> const & rhs) : state(rhs.state) {}

pointer allocate(std::size_t n)
{
    return static_cast<pointer>(::operator new(sizeof(T) * n));
}

void deallocate(pointer p, std::size_t)
{
    ::operator delete(p);
}

template <typename U, typename V>
friend bool operator==(SimpleAllocator<U> const & lhs, SimpleAllocator<V> const & rhs)
{
    return lhs.state == rhs.state;
}

template <typename U, typename V>
friend bool operator!=(SimpleAllocator<U> const & lhs, SimpleAllocator<V> const & rhs)
{
    return lhs.state != rhs.state;
}

private:
void * state;
};

using astring = std::basic_string<char, std::char_traits, SimpleAllocator>;

int main()
{
int x, y;

SimpleAllocator<void> a(&x), b(&y);

astring s("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", a);
astring t(std::move(s), b);

}

The code sample comes from #20709
The latest clang compiles the attached code sample, but gcc produces many errors.

I reported the problem to gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86074

In this discussion, programmers indicate that clang shall also reject the code.
My report was marked as duplicated with multiple reports:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78925
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54376

It said that GCC, EDG, and MSVC all reject the code, but clang++ accepts it.

Shall clang repair the problem?

[zygoloid]

*** This bug has been marked as a duplicate of bug llvm/llvm-bugzilla-archive#31507 ***