Index: clang/lib/Format/DefinitionBlockSeparator.cpp
===================================================================
--- clang/lib/Format/DefinitionBlockSeparator.cpp
+++ clang/lib/Format/DefinitionBlockSeparator.cpp
@@ -35,19 +35,31 @@
   const bool IsNeverStyle =
       Style.SeparateDefinitionBlocks == FormatStyle::SDS_Never;
   const AdditionalKeywords &ExtraKeywords = Tokens.getKeywords();
-  auto LikelyDefinition = [this, ExtraKeywords](const AnnotatedLine *Line,
-                                                bool ExcludeEnum = false) {
+  auto GetBracketLevelChange = [](const FormatToken *Tok) {
+    if (Tok->isOneOf(tok::l_brace, tok::l_paren, tok::l_square))
+      return 1;
+    if (Tok->isOneOf(tok::r_brace, tok::r_paren, tok::r_square))
+      return -1;
+    return 0;
+  };
+  auto LikelyDefinition = [&](const AnnotatedLine *Line,
+                              bool ExcludeEnum = false) {
     if ((Line->MightBeFunctionDecl && Line->mightBeFunctionDefinition()) ||
         Line->startsWithNamespace())
       return true;
-    FormatToken *CurrentToken = Line->First;
-    while (CurrentToken) {
-      if (CurrentToken->isOneOf(tok::kw_class, tok::kw_struct) ||
-          (Style.isJavaScript() && CurrentToken->is(ExtraKeywords.kw_function)))
-        return true;
-      if (!ExcludeEnum && CurrentToken->is(tok::kw_enum))
-        return true;
-      CurrentToken = CurrentToken->Next;
+    int BracketLevel = 0;
+    for (const FormatToken *CurrentToken = Line->First; CurrentToken;
+         CurrentToken = CurrentToken->Next) {
+      if (BracketLevel == 0) {
+        if ((CurrentToken->isOneOf(tok::kw_class, tok::kw_struct,
+                                   tok::kw_union) ||
+             (Style.isJavaScript() &&
+              CurrentToken->is(ExtraKeywords.kw_function))))
+          return true;
+        if (!ExcludeEnum && CurrentToken->is(tok::kw_enum))
+          return true;
+      }
+      BracketLevel += GetBracketLevelChange(CurrentToken);
     }
     return false;
   };
@@ -102,14 +114,17 @@
              IsPPConditional(OpeningLineIndex - 1);
     };
     const auto HasEnumOnLine = [&]() {
-      FormatToken *CurrentToken = CurrentLine->First;
       bool FoundEnumKeyword = false;
-      while (CurrentToken) {
-        if (CurrentToken->is(tok::kw_enum))
-          FoundEnumKeyword = true;
-        else if (FoundEnumKeyword && CurrentToken->is(tok::l_brace))
-          return true;
-        CurrentToken = CurrentToken->Next;
+      int BracketLevel = 0;
+      for (const FormatToken *CurrentToken = CurrentLine->First; CurrentToken;
+           CurrentToken = CurrentToken->Next) {
+        if (BracketLevel == 0) {
+          if (CurrentToken->is(tok::kw_enum))
+            FoundEnumKeyword = true;
+          else if (FoundEnumKeyword && CurrentToken->is(tok::l_brace))
+            return true;
+        }
+        BracketLevel += GetBracketLevelChange(CurrentToken);
       }
       return FoundEnumKeyword && I + 1 < Lines.size() &&
              Lines[I + 1]->First->is(tok::l_brace);
Index: clang/unittests/Format/DefinitionBlockSeparatorTest.cpp
===================================================================
--- clang/unittests/Format/DefinitionBlockSeparatorTest.cpp
+++ clang/unittests/Format/DefinitionBlockSeparatorTest.cpp
@@ -109,6 +109,15 @@
                "};",
                Style);
 
+  verifyFormat("union foo {\n"
+               "  int i, j;\n"
+               "};\n"
+               "\n"
+               "union bar {\n"
+               "  int j, k;\n"
+               "};",
+               Style);
+
   verifyFormat("class foo {\n"
                "  int i, j;\n"
                "};\n"
@@ -311,6 +320,9 @@
                       "int bar3(int j, int k, const enum Bar b) {\n"
                       "  // A comment\n"
                       "  int r = j % k;\n"
+                      "  if (struct S = getS()) {\n"
+                      "    // if condition\n"
+                      "  }\n"
                       "  return r;\n"
                       "}\n";
   std::string Postfix = "\n"
@@ -364,6 +376,9 @@
                         "int bar3(int j, int k, const enum Bar b) {\n"
                         "  // A comment\n"
                         "  int r = j % k;\n"
+                        "  if (struct S = getS()) {\n"
+                        "    // if condition\n"
+                        "  }\n"
                         "  return r;\n"
                         "}\n"
                         "} // namespace";
@@ -425,6 +440,10 @@
                "{\n"
                "  // A comment\n"
                "  int r = j % k;\n"
+               "  if (struct S = getS())\n"
+               "  {\n"
+               "    // if condition\n"
+               "  }\n"
                "  return r;\n"
                "}\n"
                "} // namespace NS",
@@ -473,6 +492,9 @@
                         "int bar3(int j, int k, const enum Bar b) {\n"
                         "  // A comment\n"
                         "  int r = j % k;\n"
+                        "  if (struct S = getS()) {\n"
+                        "    // if condition\n"
+                        "  }\n"
                         "  return r;\n"
                         "}\n"
                         "} // namespace";