STLExtras: Provide less/equal functors with templated function call operators, plus a deref'ing functor template utility

Similar to the C++14 void specializations of these templates, useful as
a stop-gap until LLVM switches to '14.

Example use-cases in tblgen because I saw some functors that looked like
they could be simplified/refactored.

Reviewers: dexonsmith

Differential Revision: http://reviews.llvm.org/D7324

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227828 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/STLExtras.h

@@ -24,6 +24,7 @@
 #include <iterator>
 #include <memory>
 #include <utility> // for std::pair
+#include <cassert>
 
 namespace llvm {
 
@@ -558,6 +559,35 @@ struct pair_hash {
   }
 };
 
+/// A functor like C++14's std::less<void> in its absence.
+struct less {
+  template <typename A, typename B> bool operator()(A &&a, B &&b) const {
+    return std::forward<A>(a) < std::forward<B>(b);
+  }
+};
+
+/// A functor like C++14's std::equal<void> in its absence.
+struct equal {
+  template <typename A, typename B> bool operator()(A &&a, B &&b) const {
+    return std::forward<A>(a) == std::forward<B>(b);
+  }
+};
+
+/// Binary functor that adapts to any other binary functor after dereferencing
+/// operands.
+template <typename T> struct deref {
+  T func;
+  // Could be further improved to cope with non-derivable functors and
+  // non-binary functors (should be a variadic template member function
+  // operator()).
+  template <typename A, typename B>
+  auto operator()(A &lhs, B &rhs) const -> decltype(func(*lhs, *rhs)) {
+    assert(lhs);
+    assert(rhs);
+    return func(*lhs, *rhs);
+  }
+};
+
 } // End llvm namespace
 
 #endif