Index: include/clang/Format/Format.h
===================================================================
--- include/clang/Format/Format.h
+++ include/clang/Format/Format.h
@@ -1584,6 +1584,17 @@
     ///    }
     /// \endcode
     SBPO_ControlStatements,
+    /// Put a space before opening parentheses only if the parentheses are not
+    /// empty i.e. '()'
+    /// \code
+    ///   void() {
+    ///     if (true) {
+    ///       f();
+    ///       g (x, y, z);
+    ///     }
+    ///   }
+    /// \endcode
+    SBPO_NonEmptyParentheses,
     /// Always put a space before opening parentheses, except when it's
     /// prohibited by the syntax rules (in function-like macro definitions) or
     /// when determined by other style rules (after unary operators, opening
Index: lib/Format/Format.cpp
===================================================================
--- lib/Format/Format.cpp
+++ lib/Format/Format.cpp
@@ -260,6 +260,8 @@
     IO.enumCase(Value, "Never", FormatStyle::SBPO_Never);
     IO.enumCase(Value, "ControlStatements",
                 FormatStyle::SBPO_ControlStatements);
+    IO.enumCase(Value, "NonEmptyParentheses",
+                FormatStyle::SBPO_NonEmptyParentheses);
     IO.enumCase(Value, "Always", FormatStyle::SBPO_Always);
 
     // For backward compatibility.
Index: lib/Format/TokenAnnotator.cpp
===================================================================
--- lib/Format/TokenAnnotator.cpp
+++ lib/Format/TokenAnnotator.cpp
@@ -62,7 +62,7 @@
     if (NonTemplateLess.count(CurrentToken->Previous))
       return false;
 
-    const FormatToken &Previous = *CurrentToken->Previous;  // The '<'.
+    const FormatToken &Previous = *CurrentToken->Previous; // The '<'.
     if (Previous.Previous) {
       if (Previous.Previous->Tok.isLiteral())
         return false;
@@ -366,7 +366,7 @@
       // specifier parameter, although this is technically valid:
       // [[foo(:)]]
       if (AttrTok->is(tok::colon) ||
-          AttrTok->startsSequence(tok::identifier, tok::identifier) || 
+          AttrTok->startsSequence(tok::identifier, tok::identifier) ||
           AttrTok->startsSequence(tok::r_paren, tok::identifier))
         return false;
       if (AttrTok->is(tok::ellipsis))
@@ -523,11 +523,11 @@
         // Here, we set FirstObjCSelectorName when the end of the method call is
         // reached, in case it was not set already.
         if (!Contexts.back().FirstObjCSelectorName) {
-            FormatToken* Previous = CurrentToken->getPreviousNonComment();
-            if (Previous && Previous->is(TT_SelectorName)) {
-              Previous->ObjCSelectorNameParts = 1;
-              Contexts.back().FirstObjCSelectorName = Previous;
-            }
+          FormatToken *Previous = CurrentToken->getPreviousNonComment();
+          if (Previous && Previous->is(TT_SelectorName)) {
+            Previous->ObjCSelectorNameParts = 1;
+            Contexts.back().FirstObjCSelectorName = Previous;
+          }
         } else {
           Left->ParameterCount =
               Contexts.back().FirstObjCSelectorName->ObjCSelectorNameParts;
@@ -1388,7 +1388,8 @@
         Current.Type = Current.Previous->Type;
       }
     } else if (canBeObjCSelectorComponent(Current) &&
-               // FIXME(bug 36976): ObjC return types shouldn't use TT_CastRParen.
+               // FIXME(bug 36976): ObjC return types shouldn't use
+               // TT_CastRParen.
                Current.Previous && Current.Previous->is(TT_CastRParen) &&
                Current.Previous->MatchingParen &&
                Current.Previous->MatchingParen->Previous &&
@@ -2414,9 +2415,9 @@
   if (Right.isOneOf(tok::semi, tok::comma))
     return false;
   if (Right.is(tok::less) && Line.Type == LT_ObjCDecl) {
-    bool IsLightweightGeneric =
-        Right.MatchingParen && Right.MatchingParen->Next &&
-        Right.MatchingParen->Next->is(tok::colon);
+    bool IsLightweightGeneric = Right.MatchingParen &&
+                                Right.MatchingParen->Next &&
+                                Right.MatchingParen->Next->is(tok::colon);
     return !IsLightweightGeneric && Style.ObjCSpaceBeforeProtocolList;
   }
   if (Right.is(tok::less) && Left.is(tok::kw_template))
@@ -2538,7 +2539,18 @@
               (!Left.Previous || Left.Previous->isNot(tok::period))))) ||
            (Style.SpaceBeforeParens == FormatStyle::SBPO_Always &&
             (Left.is(tok::identifier) || Left.isFunctionLikeKeyword() ||
-             Left.is(tok::r_paren)) &&
+             Left.is(tok::r_paren) ||
+             (Left.Tok.getIdentifierInfo() &&
+              Left.Tok.getIdentifierInfo()->isKeyword(
+                  getFormattingLangOpts(Style)))) &&
+            Line.Type != LT_PreprocessorDirective) ||
+           (Style.SpaceBeforeParens == FormatStyle::SBPO_NonEmptyParentheses &&
+            Right.ParameterCount >= 1 &&
+            (Left.is(tok::identifier) || Left.isFunctionLikeKeyword() ||
+             Left.is(tok::r_square) || Left.is(tok::r_paren) ||
+             (Left.Tok.getIdentifierInfo() &&
+              Left.Tok.getIdentifierInfo()->isKeyword(
+                  getFormattingLangOpts(Style)))) &&
             Line.Type != LT_PreprocessorDirective);
   }
   if (Left.is(tok::at) && Right.Tok.getObjCKeywordID() != tok::objc_not_keyword)
@@ -2601,7 +2613,8 @@
     // Slashes occur in text protocol extension syntax: [type/type] { ... }.
     if (Left.is(tok::slash) || Right.is(tok::slash))
       return false;
-    if (Left.MatchingParen && Left.MatchingParen->is(TT_ProtoExtensionLSquare) &&
+    if (Left.MatchingParen &&
+        Left.MatchingParen->is(TT_ProtoExtensionLSquare) &&
         Right.isOneOf(tok::l_brace, tok::less))
       return !Style.Cpp11BracedListStyle;
     // A percent is probably part of a formatting specification, such as %lld.
@@ -2729,7 +2742,9 @@
       Left.isOneOf(TT_TrailingReturnArrow, TT_LambdaArrow))
     return true;
   if (Right.is(TT_OverloadedOperatorLParen))
-    return Style.SpaceBeforeParens == FormatStyle::SBPO_Always;
+    return Style.SpaceBeforeParens == FormatStyle::SBPO_Always ||
+           (Style.SpaceBeforeParens == FormatStyle::SBPO_NonEmptyParentheses &&
+            Right.ParameterCount >= 1);
   if (Left.is(tok::comma))
     return true;
   if (Right.is(tok::comma))
@@ -2813,7 +2828,9 @@
     return true;
   if (Left.is(TT_TemplateCloser) && Right.is(tok::l_paren) &&
       Right.isNot(TT_FunctionTypeLParen))
-    return Style.SpaceBeforeParens == FormatStyle::SBPO_Always;
+    return Style.SpaceBeforeParens == FormatStyle::SBPO_Always ||
+           (Style.SpaceBeforeParens == FormatStyle::SBPO_NonEmptyParentheses &&
+            Right.ParameterCount >= 1);
   if (Right.is(TT_TemplateOpener) && Left.is(tok::r_paren) &&
       Left.MatchingParen && Left.MatchingParen->is(TT_OverloadedOperatorLParen))
     return false;
@@ -3118,7 +3135,7 @@
     // function f(): a is B { ... }
     // Do not break before is in these cases.
     if (Right.is(Keywords.kw_is)) {
-      const FormatToken* Next = Right.getNextNonComment();
+      const FormatToken *Next = Right.getNextNonComment();
       // If `is` is followed by a colon, it's likely that it's a dict key, so
       // ignore it for this check.
       // For example this is common in Polymer:
Index: unittests/Format/FormatTest.cpp
===================================================================
--- unittests/Format/FormatTest.cpp
+++ unittests/Format/FormatTest.cpp
@@ -9271,6 +9271,59 @@
   verifyFormat("typedef void (*cb) (int);", Space);
   verifyFormat("T A::operator() ();", Space);
   verifyFormat("X A::operator++ (T);", Space);
+  verifyFormat("int x = int (y);", Space);
+
+  FormatStyle SomeSpace = getLLVMStyle();
+  SomeSpace.SpaceBeforeParens = FormatStyle::SBPO_NonEmptyParentheses;
+
+  verifyFormat("[]() -> float {}", SomeSpace);
+  verifyFormat("[] (auto foo) {}", SomeSpace);
+  verifyFormat("[foo]() -> int {}", SomeSpace);
+  verifyFormat("int f();", SomeSpace);
+  verifyFormat("void f (int a, T b) {\n"
+               "  while (true)\n"
+               "    continue;\n"
+               "}",
+               SomeSpace);
+  verifyFormat("if (true)\n"
+               "  f();\n"
+               "else if (true)\n"
+               "  f();",
+               SomeSpace);
+  verifyFormat("do {\n"
+               "  do_something();\n"
+               "} while (something());",
+               SomeSpace);
+  verifyFormat("switch (x) {\n"
+               "default:\n"
+               "  break;\n"
+               "}",
+               SomeSpace);
+  verifyFormat("A::A() : a (1) {}", SomeSpace);
+  verifyFormat("void f() __attribute__ ((asdf));", SomeSpace);
+  verifyFormat("*(&a + 1);\n"
+               "&((&a)[1]);\n"
+               "a[(b + c) * d];\n"
+               "(((a + 1) * 2) + 3) * 4;",
+               SomeSpace);
+  verifyFormat("#define A(x) x", SomeSpace);
+  verifyFormat("#define A (x) x", SomeSpace);
+  verifyFormat("#if defined(x)\n"
+               "#endif",
+               SomeSpace);
+  verifyFormat("auto i = std::make_unique<int> (5);", SomeSpace);
+  verifyFormat("size_t x = sizeof (x);", SomeSpace);
+  verifyFormat("auto f (int x) -> decltype (x);", SomeSpace);
+  verifyFormat("int f (T x) noexcept (x.create());", SomeSpace);
+  verifyFormat("alignas (128) char a[128];", SomeSpace);
+  verifyFormat("size_t x = alignof (MyType);", SomeSpace);
+  verifyFormat("static_assert (sizeof (char) == 1, \"Impossible!\");",
+               SomeSpace);
+  verifyFormat("int f() throw (Deprecated);", SomeSpace);
+  verifyFormat("typedef void (*cb) (int);", SomeSpace);
+  verifyFormat("T A::operator()();", SomeSpace);
+  verifyFormat("X A::operator++ (T);", SomeSpace);
+  verifyFormat("int x = int (y);", SomeSpace);
 }
 
 TEST_F(FormatTest, ConfigurableSpacesInParentheses) {
@@ -11055,6 +11108,8 @@
               FormatStyle::SBPO_Always);
   CHECK_PARSE("SpaceBeforeParens: ControlStatements", SpaceBeforeParens,
               FormatStyle::SBPO_ControlStatements);
+  CHECK_PARSE("SpaceBeforeParens: NonEmptyParentheses", SpaceBeforeParens,
+              FormatStyle::SBPO_NonEmptyParentheses);
   // For backward compatibility:
   CHECK_PARSE("SpaceAfterControlStatementKeyword: false", SpaceBeforeParens,
               FormatStyle::SBPO_Never);