LWG3045 atomic<floating-point> doesn't have value_type or difference_type

Author: Dawn Perchik

source/atomics.tex

@@ -1146,6 +1146,8 @@
 \begin{codeblock}
 namespace std {
   template<> struct atomic<@\placeholder{floating-point}@> {
+    using value_type = @\placeholdernc{floating-point}@;
+    using difference_type = value_type;
     static constexpr bool is_always_lock_free = @\impdefx{whether a given \tcode{atomic} type's operations are always lock free}@;
     bool is_lock_free() const volatile noexcept;
     bool is_lock_free() const noexcept;