Index: include/clang/Format/Format.h
===================================================================
--- include/clang/Format/Format.h
+++ include/clang/Format/Format.h
@@ -112,6 +112,10 @@
   /// single line.
   bool AllowShortLoopsOnASingleLine;
 
+  /// \brief If \c true, <tt> namespace a { class b; } </tt> can be put
+  /// on a single line
+  bool AllowShortNamespacesOnASingleLine;
+
   /// \brief Different ways to break after the function definition return type.
   enum DefinitionReturnTypeBreakingStyle {
     /// Break after return type automatically.
Index: lib/Format/Format.cpp
===================================================================
--- lib/Format/Format.cpp
+++ lib/Format/Format.cpp
@@ -212,6 +212,8 @@
                    Style.AllowShortIfStatementsOnASingleLine);
     IO.mapOptional("AllowShortLoopsOnASingleLine",
                    Style.AllowShortLoopsOnASingleLine);
+    IO.mapOptional("AllowShortNamespacesOnASingleLine",
+                   Style.AllowShortNamespacesOnASingleLine);
     IO.mapOptional("AlwaysBreakAfterDefinitionReturnType",
                    Style.AlwaysBreakAfterDefinitionReturnType);
     IO.mapOptional("AlwaysBreakBeforeMultilineStrings",
@@ -354,6 +356,7 @@
   LLVMStyle.AllowShortCaseLabelsOnASingleLine = false;
   LLVMStyle.AllowShortIfStatementsOnASingleLine = false;
   LLVMStyle.AllowShortLoopsOnASingleLine = false;
+  LLVMStyle.AllowShortNamespacesOnASingleLine = false;
   LLVMStyle.AlwaysBreakAfterDefinitionReturnType = FormatStyle::DRTBS_None;
   LLVMStyle.AlwaysBreakBeforeMultilineStrings = false;
   LLVMStyle.AlwaysBreakTemplateDeclarations = false;
Index: lib/Format/UnwrappedLineFormatter.cpp
===================================================================
--- lib/Format/UnwrappedLineFormatter.cpp
+++ lib/Format/UnwrappedLineFormatter.cpp
@@ -199,6 +199,12 @@
       return MergeShortFunctions ? tryMergeSimpleBlock(I, E, Limit) : 0;
     }
     if (TheLine->Last->is(tok::l_brace)) {
+      if (Style.AllowShortNamespacesOnASingleLine &&
+         TheLine->First->is(tok::kw_namespace)) {
+        if (unsigned result = tryMergeNamespace(I, E, Limit)) {
+          return result;
+        }
+      }
       return Style.BreakBeforeBraces == FormatStyle::BS_Attach
                  ? tryMergeSimpleBlock(I, E, Limit)
                  : 0;
@@ -258,6 +264,35 @@
     return 1;
   }
 
+  unsigned tryMergeNamespace(
+      SmallVectorImpl<AnnotatedLine *>::const_iterator I,
+      SmallVectorImpl<AnnotatedLine *>::const_iterator E, unsigned Limit) {
+    if (Limit == 0)
+      return 0;
+    if (I[1]->InPPDirective != (*I)->InPPDirective ||
+        (I[1]->InPPDirective && I[1]->First->HasUnescapedNewline))
+      return 0;
+
+    Limit = limitConsideringMacros(I + 1, E, Limit);
+
+    if (1 + I[1]->Last->TotalLength > Limit)
+      return 0;
+
+    // An assumption of the function is that the first line begins with
+    // keyword namespace (i.e. I[0]->First->is(tok::kw_namespace))
+
+    // The line which is in the namespace should end with semicolon
+    if (I[1]->Last->isNot(tok::semi))
+      return 0;
+
+    // Last, check that the third line starts with a closing brace.
+    if (I[2]->First->isNot(tok::r_brace))
+      return 0;
+
+    // If so, merge all three lines.
+    return 2;
+  }
+
   unsigned tryMergeSimpleControlStatement(
       SmallVectorImpl<AnnotatedLine *>::const_iterator I,
       SmallVectorImpl<AnnotatedLine *>::const_iterator E, unsigned Limit) {
Index: unittests/Format/FormatTest.cpp
===================================================================
--- unittests/Format/FormatTest.cpp
+++ unittests/Format/FormatTest.cpp
@@ -10433,6 +10433,17 @@
   verifyNoCrash("#define a\\\n /**/}");
 }
 
+TEST_F(FormatTest, ShortNamespacesOption) {
+  FormatStyle style = getLLVMStyle();
+  style.AllowShortNamespacesOnASingleLine = true;
+
+  verifyFormat("namespace foo { class bar; }", style);
+  verifyFormat("namespace foo {\nclass bar;\nclass baz;\n}", style);
+  verifyFormat("namespace foo {\nint f() { return 5; }\n}", style);
+  verifyFormat("namespace foo { template <T> struct bar; }", style);
+  verifyFormat("namespace foo { constexpr int num = 42; }", style);
+}
+
 } // end namespace
 } // end namespace format
 } // end namespace clang