diff --git a/clang/lib/Format/UnwrappedLineFormatter.cpp b/clang/lib/Format/UnwrappedLineFormatter.cpp
--- a/clang/lib/Format/UnwrappedLineFormatter.cpp
+++ b/clang/lib/Format/UnwrappedLineFormatter.cpp
@@ -621,6 +621,10 @@
   tryMergeSimpleBlock(SmallVectorImpl<AnnotatedLine *>::const_iterator I,
                       SmallVectorImpl<AnnotatedLine *>::const_iterator E,
                       unsigned Limit) {
+    // Don't merge with a preprocessor directive.
+    if (I[1]->Type == LT_PreprocessorDirective)
+      return 0;
+
     AnnotatedLine &Line = **I;
 
     // Don't merge ObjC @ keywords and methods.
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -1805,6 +1805,21 @@
   verifyFormat("#define xor(x) (^(x))");
   verifyFormat("#define __except(x)");
   verifyFormat("#define __try(x)");
+
+  FormatStyle Style = getLLVMStyle();
+  Style.BreakBeforeBraces = FormatStyle::BS_Custom;
+  Style.BraceWrapping.AfterFunction = true;
+  // Test that a macro definition never gets merged with the following
+  // definition.
+  // FIXME: The AAA macro definition probably should not be split into 3 lines.
+  verifyFormat("#define AAA                                                    "
+               "                \\\n"
+               "  N                                                            "
+               "                \\\n"
+               "  {\n"
+               "#define BBB }\n",
+               Style);
+  // verifyFormat("#define AAA N { //\n", Style);
 }
 
 TEST_F(FormatTest, ShortBlocksInMacrosDontMergeWithCodeAfterMacro) {