Index: clang/lib/Format/TokenAnnotator.cpp
===================================================================
--- clang/lib/Format/TokenAnnotator.cpp
+++ clang/lib/Format/TokenAnnotator.cpp
@@ -3494,7 +3494,8 @@
 }
 static bool isAllmanLambdaBrace(const FormatToken &Tok) {
   return (Tok.is(tok::l_brace) && Tok.is(BK_Block) &&
-          !Tok.isOneOf(TT_ObjCBlockLBrace, TT_DictLiteral));
+          !Tok.isOneOf(TT_ObjCBlockLBrace, TT_DictLiteral) &&
+          !Tok.Previous->Previous->is(tok::kw_namespace));
 }
 
 static bool isAllmanBraceIncludedBreakableLambda(
Index: clang/unittests/Format/FormatTest.cpp
===================================================================
--- clang/unittests/Format/FormatTest.cpp
+++ clang/unittests/Format/FormatTest.cpp
@@ -2589,6 +2589,25 @@
                    Style));
 }
 
+TEST_F(FormatTest, FormatsCompactNamespacesLambdaRegression) {
+  // Make sure compact namespaces are not confused with lambdas
+  FormatStyle CompactNamespacesStyle{getLLVMStyle()};
+  CompactNamespacesStyle.CompactNamespaces = true;
+  CompactNamespacesStyle.AllowShortLambdasOnASingleLine = FormatStyle::SLS_None;
+  CompactNamespacesStyle.BreakBeforeBraces = FormatStyle::BS_Custom;
+  CompactNamespacesStyle.BraceWrapping.BeforeLambdaBody = true;
+  verifyFormat("namespace out { namespace in {\n"
+               "}} // namespace out::in",
+               CompactNamespacesStyle);
+  EXPECT_EQ("namespace out { namespace in {\n"
+            "}} // namespace out::in",
+            format("namespace out {\n"
+                   "namespace in {\n"
+                   "} // namespace in\n"
+                   "} // namespace out",
+                   CompactNamespacesStyle));
+}
+
 TEST_F(FormatTest, FormatsExternC) {
   verifyFormat("extern \"C\" {\nint a;");
   verifyFormat("extern \"C\" {}");