[libcxx-commits] [libcxx] [libc++] std::atomic primary template should not have a `difference_type` member type (PR #123236)

[Louis Dionne via libcxx-commits]

================
@@ -29,163 +29,176 @@
 #  include <thread>
 #endif
 
-template <class A, bool Integral>
-struct test_atomic
-{
-    test_atomic()
-    {
-        A a; (void)a;
+// detect existence of the difference_type member type
+template <class...>
+using myvoid_t = void;
+template <typename T, typename = void>
+struct has_difference_type : std::false_type {};
+template <typename T>
+struct has_difference_type<T, myvoid_t<typename T::difference_type> > : std::true_type {};
+
+template <class A, bool IntegralOrFloating, bool Pointer>
----------------
ldionne wrote:

```suggestion
template <class T, bool IntegralOrFloating = (std::is_integral<T>::value && !std::is_same<T, bool>::value) || std::is_floating_point<T>::value, bool Pointer = std::is_pointer<T>::value>
```

We can take `T` directly here while we're at it, and then form `using A = std::atomic<T>`. That simplifies a few things.

https://github.com/llvm/llvm-project/pull/123236