is_integral_or_enum ❥ enum class ⇒ hashable enum class

Summary:
As discussed in D18775 making AtomicOrdering an enum class makes it non-hashable, which shouldn't be the case. Hashing.h defines hash_value for all is_integral_or_enum, but type_traits.h's definition of is_integral_or_enum only checks for *inplicit* conversion to integral types which leaves enum classes out and is very confusing because is_enum is true for enum classes.

This patch:
  - Adds a check for is_enum when determining is_integral_or_enum.
  - Explicitly converts the value parameter in hash_value to handle enum class hashing.

Note that the warning at the top of Hashing.h still applies: each execution of the program has a high probability of producing a different hash_code for a given input. Thus their values are not stable to save or persist, and should only be used during the execution for the construction of hashing datastructures.

Reviewers: dberlin, chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18938

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265879 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/Hashing.h

@@ -632,7 +632,8 @@ inline hash_code hash_integer_value(uint64_t value) {
 template <typename T>
 typename std::enable_if<is_integral_or_enum<T>::value, hash_code>::type
 hash_value(T value) {
-  return ::llvm::hashing::detail::hash_integer_value(value);
+  return ::llvm::hashing::detail::hash_integer_value(
+      static_cast<typename std::underlying_type<T>::type>(value));
 }
 
 // Declared and documented above, but defined here so that any of the hashing

include/llvm/Support/type_traits.h

@@ -54,11 +54,12 @@ struct isPodLike<std::pair<T, U> > {
 };
 
 /// \brief Metafunction that determines whether the given type is either an
-/// integral type or an enumeration type.
+/// integral type or an enumeration type, including enum classes.
 ///
 /// Note that this accepts potentially more integral types than is_integral
-/// because it is based on merely being convertible implicitly to an integral
-/// type.
+/// because it is based on being implicitly convertible to an integral type.
+/// Also note that enum classes aren't implicitly convertible to integral types,
+/// the value may therefore need to be explicitly converted before being used.
 template <typename T> class is_integral_or_enum {
   typedef typename std::remove_reference<T>::type UnderlyingT;
 
@@ -67,7 +68,8 @@ template <typename T> class is_integral_or_enum {
       !std::is_class<UnderlyingT>::value && // Filter conversion operators.
       !std::is_pointer<UnderlyingT>::value &&
       !std::is_floating_point<UnderlyingT>::value &&
-      std::is_convertible<UnderlyingT, unsigned long long>::value;
+      (std::is_enum<UnderlyingT>::value ||
+       std::is_convertible<UnderlyingT, unsigned long long>::value);
 };
 
 /// \brief If T is a pointer, just return it. If it is not, return T&.