Index: clang/lib/Format/TokenAnnotator.h
===================================================================
--- clang/lib/Format/TokenAnnotator.h
+++ clang/lib/Format/TokenAnnotator.h
@@ -99,9 +99,21 @@
   /// function declaration. Asserts MightBeFunctionDecl.
   bool mightBeFunctionDefinition() const {
     assert(MightBeFunctionDecl);
-    // FIXME: Line.Last points to other characters than tok::semi
-    // and tok::lbrace.
-    return !Last->isOneOf(tok::semi, tok::comment);
+    // Try to determine if the end of a stream of tokens is either the
+    // Definition or the Declaration for a function. It does this by looking for
+    // the ';' in foo(); and using that it ends with a ; to know this is the
+    // Definition, however the line could end with
+    //    foo(); /* comment */
+    // or
+    //    foo(); // comment
+    // or
+    //    foo() // comment
+    // In all cases the last token would be a comment (old way) but a definition
+    // like the last one means you must look at the last two tokens to
+    // ensure its not `; <comment>`.
+    return Last->isNot(tok::semi) &&
+           (Last->Previous &&
+            !Last->Previous->startsSequence(tok::semi, tok::comment));
   }
 
   /// \c true if this line starts a namespace definition.
Index: clang/unittests/Format/FormatTest.cpp
===================================================================
--- clang/unittests/Format/FormatTest.cpp
+++ clang/unittests/Format/FormatTest.cpp
@@ -5745,6 +5745,72 @@
                "  return a;\n"
                "}\n",
                Style);
+
+  Style = getGNUStyle();
+  // Test for comments at the define of function definitions and declarations.
+
+  verifyFormat("void\n"
+               "foo (int a, /*abc*/ int b // def\n"
+               ")\n"
+               "{\n"
+               "}\n",
+               Style);
+
+  verifyFormat("void\n"
+               "foo (int a, /*abc*/ int b) // def\n"
+               "{\n"
+               "}\n",
+               Style);
+
+  verifyFormat("void\n"
+               "foo (int a, // abc\n"
+               "     int b) // def\n"
+               "{\n"
+               "}\n",
+               Style);
+
+  verifyFormat("void\n"
+               "foo (int a, /*abc*/ int b)\n"
+               "{\n"
+               "}\n",
+               Style);
+
+  verifyFormat("void\n"
+               "foo (int a, /* abc */ int b) /* def */\n"
+               "{\n"
+               "}\n",
+               Style);
+
+  verifyFormat("void\n"
+               "foo (int a, /*abc*/ int /*def=*/b)\n"
+               "{\n"
+               "}\n",
+               Style);
+
+  verifyFormat("void\n"
+               "foo (int a, /*abc*/ int /*def=*/b)\n"
+               "{\n"
+               "}\n",
+               Style);
+
+  // Definitions that should not break after return type
+  verifyFormat("void foo (int a, // abc\n"
+               "          int /*def=*/b);\n",
+               Style);
+  verifyFormat("void foo (int a,          // abc\n"
+               "          int /*def=*/b); /* def */\n",
+               Style);
+  verifyFormat("void foo (int a,          // abc\n"
+               "          int /*def=*/b); // def\n",
+               Style);
+
+  verifyFormat("void foo (int a,  // abc\n"
+               "          int b); // def\n",
+               Style);
+
+  verifyFormat("void foo (int a, int b); // def\n", Style);
+
+  verifyFormat("void foo (int a, int b); /* def */\n", Style);
 }
 
 TEST_F(FormatTest, AlwaysBreakBeforeMultilineStrings) {