[libcxx] [test] Add TEST_STACK_ALLOCATOR_WORKAROUND.

[STL_MSFT]

Summary

Add TEST_STACK_ALLOCATOR_WORKAROUND. As I reported to Eric and Marshall:

"stack_allocator<T, N> is seriously nonconformant to N4582 17.6.3.5 [allocator.requirements].

First, it lacks a rebinding constructor. (The nested "struct rebind" isn't sufficient.)

Second, it lacks templated equality/inequality.

Third, it completely ignores alignment.

Finally, and most severely, the Standard forbids its existence. Allocators are forbidden from returning memory "inside themselves". This requirement is implied by the Standard's requirements for rebinding and equality. It's permitted to return memory from a separate buffer object on the stack, though."

I would like to be able to run libcxx's tests without any local patches (I can inject macros etc. via a force-included header in a separate directory). I'm down to one local patch dealing with stack_allocator. Adding this workaround macro will allow me to consume libcxx's tests unchanged.

[STL_MSFT]

I know that this stack_allocator issue is a huge headache, but would it be possible to accept this small, non-intrusive diff upstream? This is the last local change that I have.

[EricWF]

I'm OK accepting this, but I'm going to weak it before committing so that test_macros.h define TEST_STACK_ALLOCATOR_WORKAROUND for non-libc++ builds.

[EricWF]

Nevermind. I decided to just fix stack_allocator so this patch is no longer needed. Please let me know if the stack allocator fix doesn't work

[STL_MSFT]

Abandoning, as this machinery has been replaced.

test/std/containers/stack_allocator.h

 //===----------------------------------------------------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is dual licensed under the MIT and the University of Illinois Open
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef STACK_ALLOCATOR_H
 #define STACK_ALLOCATOR_H
 
 #include <cstddef>
 #include <new>
 
+#ifdef TEST_STACK_ALLOCATOR_WORKAROUND
+
+#include <memory>
+
+template <class T, std::size_t N>
+using stack_allocator = std::allocator<T>;
+
+// This is a temporary workaround for library implementations
+// that can't tolerate the nonconformant stack_allocator below.
+
+#else  // TEST_STACK_ALLOCATOR_WORKAROUND
+
 template <class T, std::size_t N>
 class stack_allocator
 {
     char buf_[sizeof(T)*N];
     char* ptr_;
 public:
     typedef T                 value_type;
     typedef value_type*       pointer;
#endif
     size_type max_size() const {return N;}
 };
 
 template <class T, std::size_t N>
 inline
 void
 swap(stack_allocator<T, N>& x, stack_allocator<T, N>& y) {}
 
+#endif  // TEST_STACK_ALLOCATOR_WORKAROUND
+
 #endif  // STACK_ALLOCATOR_H