Reland #2 "[STLExtras] Add a template for detecting whether a type has an equality comparison operator" (llvm#177415)

Reland PR llvm#176893 which was an attempt to reland PR llvm#176429.

Fix:

There was a test case asserting that for types `StructA` `StructB` where
`operator==(const StructB &, const StructA &)` is defined,
`has_equality_comparison_v<StructA, StructB>` is false because the
arguments are the wrong way around. However, in C++20, operator overload
resolution was changed so that for reflexive comparison operators `==`
and `!=` if a candidate exists with the arguments swapped then this will
be used. This means the test case failed when compiled with C++20. That
check was simply removed in this version.

Author: BStott6

llvm/include/llvm/ADT/STLExtras.h

@@ -2692,6 +2692,17 @@ template <typename T> using has_sizeof = decltype(sizeof(T));
 template <typename T>
 constexpr bool is_incomplete_v = !is_detected<detail::has_sizeof, T>::value;
 
+// Detect types with equality comparison operators.
+namespace detail {
+template <typename T, typename U>
+using has_equality_comparison =
+    decltype(std::declval<const T &>() == std::declval<const U &>());
+} // namespace detail
+
+/// Detects when type `const T` can be compared for equality with `const U`.
+template <typename T, typename U = T>
+constexpr bool has_equality_comparison_v =
+    is_detected<detail::has_equality_comparison, T, U>::value;
 } // end namespace llvm
 
 namespace std {

llvm/unittests/ADT/STLExtrasTest.cpp

@@ -1807,6 +1807,32 @@ struct Bar {};
 static_assert(is_incomplete_v<Foo>, "Foo is incomplete");
 static_assert(!is_incomplete_v<Bar>, "Bar is defined");
 
+TEST(STLExtrasTest, HasEqualityComparison) {
+  struct NoEqualityComparison {};
+  static_assert(!has_equality_comparison_v<NoEqualityComparison>);
+
+  // Mutating equality comparison doesn't count.
+  struct MutatingEqualityComparison {
+    bool operator==(MutatingEqualityComparison &Other) { return false; }
+  };
+  static_assert(!has_equality_comparison_v<MutatingEqualityComparison>);
+
+  struct PublicEqualityComparison {
+    bool operator==(const PublicEqualityComparison &Other) const {
+      return false;
+    }
+  };
+  static_assert(has_equality_comparison_v<PublicEqualityComparison>);
+
+  struct StructA {};
+  struct StructB {
+    bool operator==(const StructA &Other) const { return false; }
+  };
+  static_assert(has_equality_comparison_v<StructB, StructA>);
+
+  SUCCEED();
+}
+
 TEST(STLExtrasTest, Search) {
   // Test finding a subsequence in the middle.
   std::vector<int> Haystack = {1, 2, 3, 4, 5, 6, 7, 8};