[ADT] Move llvm::identity into IndexedMap (NFC)

This patch moves llvm::identity to IndexedMap for two reasons:

- llvm::identity is used only IndexedMap.
- llvm::identity is not suitable for general use as it is not quite
  the same as std::identity despite the comments in identity.h.

Also, this patch renames the class to IdentityIndex and places it in
the "detail" namespace.

Author: kazutakahirata

llvm/include/llvm/ADT/IndexedMap.h

@@ -20,14 +20,25 @@
 #ifndef LLVM_ADT_INDEXEDMAP_H
 #define LLVM_ADT_INDEXEDMAP_H
 
-#include "llvm/ADT/STLForwardCompat.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallVector.h"
 #include <cassert>
 
 namespace llvm {
 
-template <typename T, typename ToIndexT = llvm::identity_cxx20>
+namespace detail {
+template <class Ty> struct IdentityIndex {
+  using is_transparent = void;
+  using argument_type = Ty;
+
+  Ty &operator()(Ty &self) const { return self; }
+  const Ty &operator()(const Ty &self) const { return self; }
+};
+} // namespace detail
+
+template <typename T, typename ToIndexT = detail::IdentityIndex<unsigned>>
 class IndexedMap {
+  using IndexT = typename ToIndexT::argument_type;
   // Prefer SmallVector with zero inline storage over std::vector. IndexedMaps
   // can grow very large and SmallVector grows more efficiently as long as T
   // is trivially copyable.
@@ -42,17 +53,14 @@ class IndexedMap {
 
   explicit IndexedMap(const T &val) : nullVal_(val) {}
 
-  template <typename IndexT> typename StorageT::reference operator[](IndexT n) {
-    unsigned Index = toIndex_(n);
-    assert(Index < storage_.size() && "index out of bounds!");
-    return storage_[Index];
+  typename StorageT::reference operator[](IndexT n) {
+    assert(toIndex_(n) < storage_.size() && "index out of bounds!");
+    return storage_[toIndex_(n)];
   }
 
-  template <typename IndexT>
   typename StorageT::const_reference operator[](IndexT n) const {
-    unsigned Index = toIndex_(n);
-    assert(Index < storage_.size() && "index out of bounds!");
-    return storage_[Index];
+    assert(toIndex_(n) < storage_.size() && "index out of bounds!");
+    return storage_[toIndex_(n)];
   }
 
   void reserve(typename StorageT::size_type s) { storage_.reserve(s); }
@@ -61,16 +69,13 @@ class IndexedMap {
 
   void clear() { storage_.clear(); }
 
-  template <typename IndexT> void grow(IndexT n) {
+  void grow(IndexT n) {
     unsigned NewSize = toIndex_(n) + 1;
     if (NewSize > storage_.size())
       resize(NewSize);
   }
 
-  template <typename IndexT> bool inBounds(IndexT n) const {
-    unsigned Index = toIndex_(n);
-    return Index < storage_.size();
-  }
+  bool inBounds(IndexT n) const { return toIndex_(n) < storage_.size(); }
 
   typename StorageT::size_type size() const { return storage_.size(); }
 };

llvm/include/llvm/ADT/identity.h

@@ -18,7 +18,6 @@
 #include "llvm/ADT/PointerIntPair.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/SparseMultiSet.h"
-#include "llvm/ADT/identity.h"
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/CodeGen/LiveRegUnits.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"