[libcxx] [test] Silence another occurrence of MSVC's spurious unused warning.

[STL_MSFT]

Summary

Silence another occurrence of MSVC's spurious unused warning.

Again, I'm sorry about this one (VSO#188582 tracks the need to fix the compiler), but this targeted suppression helps us run libcxx's tests.

[majnemer]

Do we have a way to keep track of this change so we can remove it when MSVC gets fixed?

[EricWF]

In D22818#496220, @majnemer wrote:

Do we have a way to keep track of this change so we can remove it when MSVC gets fixed?

No, but I'm not concerned about a void cast in the test suite.

[EricWF]

r277572.

test/support/test_allocator.h

public:
     { ::new((void*)p) Tag_X(Ctor_Tag{}, std::forward<Args>(args)...); }
 
     template<typename U, typename... Args>
     void construct(U* p, Args&&... args)
     { ::new((void*)p) U(std::forward<Args>(args)...); }
 
     template<typename U, typename... Args>
     void destroy(U* p)
-    { p->~U(); }
+    {
+        p->~U();
+        ((void)p); // Prevent MSVC's spurious unused warning
+    }
 };
 
 template<typename T, typename U>
 bool
 operator==(const TaggingAllocator<T>&, const TaggingAllocator<U>&)
 { return true; }
 
 template<typename T, typename U>
 bool
 operator!=(const TaggingAllocator<T>&, const TaggingAllocator<U>&)
 { return false; }
 #endif
 
 
 #endif  // TEST_ALLOCATOR_H