Index: include/clang/AST/ASTContext.h
===================================================================
--- include/clang/AST/ASTContext.h
+++ include/clang/AST/ASTContext.h
@@ -776,6 +776,10 @@
   /// without looking into cache.
   RawComment *getRawCommentForDeclNoCache(const Decl *D) const;
 
+  /// \brief Return the documentation comment attached to a given declaration,
+  /// adding it to the cache if necessary.
+  RawCommentAndCacheFlags &getRawCommentForDeclCached(const Decl *D) const;
+
 public:
   RawCommentList &getRawCommentList() {
     return Comments;
@@ -796,7 +800,8 @@
   getRawCommentForAnyRedecl(const Decl *D,
                             const Decl **OriginalDecl = nullptr) const;
 
-  /// Return parsed documentation comment attached to a given declaration.
+  /// Return parsed documentation comment attached to a given declaration, its
+  /// redeclarations, or its bases.
   /// Returns nullptr if no comment is attached.
   ///
   /// \param PP the Preprocessor used with this TU.  Could be nullptr if
Index: lib/AST/ASTContext.cpp
===================================================================
--- lib/AST/ASTContext.cpp
+++ lib/AST/ASTContext.cpp
@@ -379,53 +379,54 @@
   return D;
 }
 
-const RawComment *ASTContext::getRawCommentForAnyRedecl(
-                                                const Decl *D,
-                                                const Decl **OriginalDecl) const {
+ASTContext::RawCommentAndCacheFlags &
+ASTContext::getRawCommentForDeclCached(const Decl *D) const {
   D = adjustDeclToTemplate(D);
 
   // Check whether we have cached a comment for this declaration already.
-  {
-    llvm::DenseMap<const Decl *, RawCommentAndCacheFlags>::iterator Pos =
-        RedeclComments.find(D);
-    if (Pos != RedeclComments.end()) {
-      const RawCommentAndCacheFlags &Raw = Pos->second;
-      if (Raw.getKind() != RawCommentAndCacheFlags::NoCommentInDecl) {
-        if (OriginalDecl)
-          *OriginalDecl = Raw.getOriginalDecl();
-        return Raw.getRaw();
-      }
-    }
+  auto ItInserted = RedeclComments.insert({D, RawCommentAndCacheFlags()});
+  RawCommentAndCacheFlags& Raw = ItInserted.first->second;
+  if (!ItInserted.second)
+    return Raw;
+
+  // 'Raw' is a new entry in the cache. Populate it with any comments for the
+  // local definition.
+  Raw.setOriginalDecl(D);
+  const RawComment *RC = getRawCommentForDeclNoCache(D);
+  if (!RC) {
+    Raw.setKind(RawCommentAndCacheFlags::NoCommentInDecl);
+    return Raw;
+  }
+
+  // If we found a comment, it should be a documentation comment.
+  assert(RC->isDocumentation() || LangOpts.CommentOpts.ParseAllComments);
+  // Call order swapped to work around ICE in VS2015 RTM (Release Win32)
+  // https://connect.microsoft.com/VisualStudio/feedback/details/1741530
+  Raw.setKind(RawCommentAndCacheFlags::FromDecl);
+  Raw.setRaw(RC);
+  return Raw;
+}
+
+const RawComment *
+ASTContext::getRawCommentForAnyRedecl(const Decl *D,
+                                      const Decl **OriginalDecl) const {
+  RawCommentAndCacheFlags &OrigRaw = getRawCommentForDeclCached(D);
+  if (OrigRaw.getKind() != RawCommentAndCacheFlags::NoCommentInDecl) {
+    if (OriginalDecl)
+      *OriginalDecl = OrigRaw.getOriginalDecl();
+    return OrigRaw.getRaw();
   }
 
   // Search for comments attached to declarations in the redeclaration chain.
   const RawComment *RC = nullptr;
+
   const Decl *OriginalDeclForRC = nullptr;
   for (auto I : D->redecls()) {
-    llvm::DenseMap<const Decl *, RawCommentAndCacheFlags>::iterator Pos =
-        RedeclComments.find(I);
-    if (Pos != RedeclComments.end()) {
-      const RawCommentAndCacheFlags &Raw = Pos->second;
-      if (Raw.getKind() != RawCommentAndCacheFlags::NoCommentInDecl) {
-        RC = Raw.getRaw();
-        OriginalDeclForRC = Raw.getOriginalDecl();
-        break;
-      }
-    } else {
-      RC = getRawCommentForDeclNoCache(I);
-      OriginalDeclForRC = I;
-      RawCommentAndCacheFlags Raw;
-      if (RC) {
-        // Call order swapped to work around ICE in VS2015 RTM (Release Win32)
-        // https://connect.microsoft.com/VisualStudio/feedback/details/1741530
-        Raw.setKind(RawCommentAndCacheFlags::FromDecl);
-        Raw.setRaw(RC);
-      } else
-        Raw.setKind(RawCommentAndCacheFlags::NoCommentInDecl);
-      Raw.setOriginalDecl(I);
-      RedeclComments[I] = Raw;
-      if (RC)
-        break;
+    RawCommentAndCacheFlags &Raw = getRawCommentForDeclCached(I);
+    if (Raw.getKind() != RawCommentAndCacheFlags::NoCommentInDecl) {
+      RC = Raw.getRaw();
+      OriginalDeclForRC = Raw.getOriginalDecl();
+      break;
     }
   }
 
@@ -487,9 +488,9 @@
   return RC ? RC->parse(*this, nullptr, D) : nullptr;
 }
 
-comments::FullComment *ASTContext::getCommentForDecl(
-                                              const Decl *D,
-                                              const Preprocessor *PP) const {
+comments::FullComment *
+ASTContext::getCommentForDecl(const Decl *D,
+                                    const Preprocessor *PP) const {
   if (D->isInvalidDecl())
     return nullptr;
   D = adjustDeclToTemplate(D);