Index: lib/AST/RawCommentList.cpp
===================================================================
--- lib/AST/RawCommentList.cpp
+++ lib/AST/RawCommentList.cpp
@@ -64,10 +64,43 @@
 }
 } // unnamed namespace
 
+/// \brief Determines whether there is only whitespace in `Buffer` between `P`
+/// and the previous line.
+/// \param Buffer The buffer to search in.
+/// \param P The offset from the beginning of `Buffer` to start from.
+/// \return true if all of the characters in `Buffer` ranging from the closest
+/// line-ending character before `P` (or the beginning of `Buffer`) to `P - 1`
+/// are whitespace.
+static bool onlyWhitespaceOnLineBefore(const char *Buffer, unsigned P) {
+  // Search backwards until we see linefeed or carriage return.
+  for (unsigned I = P; I != 0; --I) {
+    switch (Buffer[I - 1]) {
+    default:
+      return false;
+    case ' ':
+    case '\t':
+    case '\f':
+    case '\v':
+      break;
+    case '\r':
+    case '\n':
+      return true;
+    }
+  }
+  // We hit the beginning of the buffer.
+  return true;
+}
+
+/// Returns whether `K` is an ordinary comment kind.
+static bool isOrdinaryKind(RawComment::CommentKind K) {
+  return (K == RawComment::RCK_OrdinaryBCPL) ||
+         (K == RawComment::RCK_OrdinaryC);
+}
+
 RawComment::RawComment(const SourceManager &SourceMgr, SourceRange SR,
                        bool Merged, bool ParseAllComments) :
     Range(SR), RawTextValid(false), BriefTextValid(false),
-    IsAttached(false), IsAlmostTrailingComment(false),
+    IsAttached(false), IsTrailingComment(false), IsAlmostTrailingComment(false),
     ParseAllComments(ParseAllComments) {
   // Extract raw comment text, if possible.
   if (SR.getBegin() == SR.getEnd() || getRawText(SourceMgr).empty()) {
@@ -75,17 +108,34 @@
     return;
   }
 
+  // Guess comment kind.
+  std::pair<CommentKind, bool> K = getCommentKind(RawText, ParseAllComments);
+
+  // Guess whether an ordinary comment is trailing.
+  if (ParseAllComments && isOrdinaryKind(K.first)) {
+    FileID BeginFileID;
+    unsigned BeginOffset;
+    std::tie(BeginFileID, BeginOffset) =
+        SourceMgr.getDecomposedLoc(Range.getBegin());
+    if (BeginOffset != 0) {
+      bool Invalid = false;
+      const char *Buffer =
+          SourceMgr.getBufferData(BeginFileID, &Invalid).data();
+      IsTrailingComment |=
+          (!Invalid && !onlyWhitespaceOnLineBefore(Buffer, BeginOffset));
+    }
+  }
+
   if (!Merged) {
-    // Guess comment kind.
-    std::pair<CommentKind, bool> K = getCommentKind(RawText, ParseAllComments);
     Kind = K.first;
-    IsTrailingComment = K.second;
+    IsTrailingComment |= K.second;
 
     IsAlmostTrailingComment = RawText.startswith("//<") ||
                                  RawText.startswith("/*<");
   } else {
     Kind = RCK_Merged;
-    IsTrailingComment = mergedCommentIsTrailingComment(RawText);
+    IsTrailingComment =
+        IsTrailingComment || mergedCommentIsTrailingComment(RawText);
   }
 }
 
@@ -239,9 +289,17 @@
   const RawComment &C2 = RC;
 
   // Merge comments only if there is only whitespace between them.
-  // Can't merge trailing and non-trailing comments.
+  // Can't merge trailing and non-trailing comments unless the second is
+  // non-trailing ordinary, as in the case:
+  //   int x; // documents x
+  //          // more text
+  // versus:
+  //   int x; // documents x
+  //   int y; // documents y
   // Merge comments if they are on same or consecutive lines.
-  if (C1.isTrailingComment() == C2.isTrailingComment() &&
+  if ((C1.isTrailingComment() == C2.isTrailingComment() ||
+       (C1.isTrailingComment() && !C2.isTrailingComment() &&
+        isOrdinaryKind(C2.getKind()))) &&
       onlyWhitespaceBetween(SourceMgr, C1.getLocEnd(), C2.getLocStart(),
                             /*MaxNewlinesAllowed=*/1)) {
     SourceRange MergedRange(C1.getLocStart(), C2.getLocEnd());
Index: test/Index/parse-all-comments.c
===================================================================
--- test/Index/parse-all-comments.c
+++ test/Index/parse-all-comments.c
@@ -33,6 +33,32 @@
 // WITH EMPTY LINE
 void multi_line_comment_empty_line(int);
 
+int notdoxy7; // Not a Doxygen juxtaposed comment.  notdoxy7 NOT_DOXYGEN
+int notdoxy8; // Not a Doxygen juxtaposed comment.  notdoxy8 NOT_DOXYGEN
+
+int trdoxy9;  /// A Doxygen non-trailing comment.  trdoxyA IS_DOXYGEN_SINGLE
+int trdoxyA;
+
+int trdoxyB;  // Not a Doxygen trailing comment.  PART_ONE
+              // It's a multiline one too.  trdoxyB NOT_DOXYGEN
+int trdoxyC;
+
+int trdoxyD;  // Not a Doxygen trailing comment.   trdoxyD NOT_DOXYGEN
+              /// This comment doesn't get merged.   trdoxyE IS_DOXYGEN
+int trdoxyE;
+
+int trdoxyF;  /// A Doxygen non-trailing comment that gets dropped on the floor.
+              // This comment will also be dropped.
+int trdoxyG;  // This one won't.  trdoxyG NOT_DOXYGEN
+
+int trdoxyH;  ///< A Doxygen trailing comment.  PART_ONE
+              // This one gets merged with it.  trdoxyH SOME_DOXYGEN
+int trdoxyI;  // This one doesn't.  trdoxyI NOT_DOXYGEN
+
+int trdoxyJ;  // Not a Doxygen trailing comment.  PART_ONE
+              ///< This one gets merged with it.  trdoxyJ SOME_DOXYGEN
+int trdoxyK;  // This one doesn't.  trdoxyK NOT_DOXYGEN
+
 #endif
 
 // RUN: rm -rf %t
@@ -60,3 +86,17 @@
 // CHECK: parse-all-comments.c:22:6: FunctionDecl=isdoxy6:{{.*}} isdoxy6 IS_DOXYGEN_SINGLE
 // CHECK: parse-all-comments.c:29:6: FunctionDecl=multi_line_comment_plus_ordinary:{{.*}} BLOCK_ORDINARY_COMMENT {{.*}} ORDINARY COMMENT {{.*}} IS_DOXYGEN_START {{.*}} IS_DOXYGEN_END
 // CHECK: parse-all-comments.c:34:6: FunctionDecl=multi_line_comment_empty_line:{{.*}} MULTILINE COMMENT{{.*}}\n{{.*}}\n{{.*}} WITH EMPTY LINE
+// CHECK: parse-all-comments.c:36:5: VarDecl=notdoxy7:{{.*}} notdoxy7 NOT_DOXYGEN
+// CHECK: parse-all-comments.c:37:5: VarDecl=notdoxy8:{{.*}} notdoxy8 NOT_DOXYGEN
+// CHECK-NOT: parse-all-comments.c:39:5: VarDecl=trdoxy9:{{.*}} trdoxyA IS_DOXYGEN_SINGLE
+// CHECK: parse-all-comments.c:40:5: VarDecl=trdoxyA:{{.*}} trdoxyA IS_DOXYGEN_SINGLE
+// CHECK: parse-all-comments.c:42:5: VarDecl=trdoxyB:{{.*}} PART_ONE {{.*}} trdoxyB NOT_DOXYGEN
+// CHECK-NOT: parse-all-comments.c:44:5: VarDecl=trdoxyC:{{.*}} trdoxyB NOT_DOXYGEN
+// CHECK: parse-all-comments.c:46:5: VarDecl=trdoxyD:{{.*}} trdoxyD NOT_DOXYGEN
+// CHECK: parse-all-comments.c:48:5: VarDecl=trdoxyE:{{.*}} trdoxyE IS_DOXYGEN
+// CHECK-NOT: parse-all-comments.c:50:5: VarDecl=trdoxyF:{{.*}} RawComment
+// CHECK: parse-all-comments.c:52:5: VarDecl=trdoxyG:{{.*}} trdoxyG NOT_DOXYGEN
+// CHECK: parse-all-comments.c:54:5: VarDecl=trdoxyH:{{.*}} PART_ONE {{.*}} trdoxyH SOME_DOXYGEN
+// CHECK: parse-all-comments.c:56:5: VarDecl=trdoxyI:{{.*}} trdoxyI NOT_DOXYGEN
+// CHECK: parse-all-comments.c:58:5: VarDecl=trdoxyJ:{{.*}} PART_ONE {{.*}} trdoxyJ SOME_DOXYGEN
+// CHECK: parse-all-comments.c:60:5: VarDecl=trdoxyK:{{.*}} trdoxyK NOT_DOXYGEN