Index: lib/Format/ContinuationIndenter.cpp
===================================================================
--- lib/Format/ContinuationIndenter.cpp
+++ lib/Format/ContinuationIndenter.cpp
@@ -453,7 +453,8 @@
   State.Column += Spaces;
   if (Current.isNot(tok::comment) && Previous.is(tok::l_paren) &&
       Previous.Previous &&
-      Previous.Previous->isOneOf(tok::kw_if, tok::kw_for)) {
+      (Previous.Previous->isOneOf(tok::kw_if, tok::kw_for) ||
+       Previous.Previous->endsSequence(tok::kw_constexpr, tok::kw_if))) {
     // Treat the condition inside an if as if it was a second function
     // parameter, i.e. let nested calls have a continuation indent.
     State.Stack.back().LastSpace = State.Column;
Index: lib/Format/TokenAnnotator.cpp
===================================================================
--- lib/Format/TokenAnnotator.cpp
+++ lib/Format/TokenAnnotator.cpp
@@ -145,6 +145,7 @@
         (Left->Previous->isOneOf(tok::kw_static_assert, tok::kw_decltype,
                                  tok::kw_if, tok::kw_while, tok::l_paren,
                                  tok::comma) ||
+         Left->Previous->endsSequence(tok::kw_constexpr, tok::kw_if) ||
          Left->Previous->is(TT_BinaryOperator))) {
       // static_assert, if and while usually contain expressions.
       Contexts.back().IsExpression = true;
@@ -572,6 +573,8 @@
       break;
     case tok::kw_if:
     case tok::kw_while:
+      if (Tok->is(tok::kw_if) && CurrentToken && CurrentToken->is(tok::kw_constexpr))
+        next();
       if (CurrentToken && CurrentToken->is(tok::l_paren)) {
         next();
         if (!parseParens(/*LookForDecls=*/true))
@@ -2060,7 +2063,8 @@
       Style.AlignAfterOpenBracket != FormatStyle::BAS_DontAlign)
     return 100;
   if (Left.is(tok::l_paren) && Left.Previous &&
-      Left.Previous->isOneOf(tok::kw_if, tok::kw_for))
+      (Left.Previous->isOneOf(tok::kw_if, tok::kw_for)
+       || Left.Previous->endsSequence(tok::kw_constexpr, tok::kw_if)))
     return 1000;
   if (Left.is(tok::equal) && InFunctionDecl)
     return 110;
@@ -2211,6 +2215,7 @@
             (Left.isOneOf(tok::kw_if, tok::pp_elif, tok::kw_for, tok::kw_while,
                           tok::kw_switch, tok::kw_case, TT_ForEachMacro,
                           TT_ObjCForIn) ||
+             Left.endsSequence(tok::kw_constexpr, tok::kw_if) ||
              (Left.isOneOf(tok::kw_try, Keywords.kw___except, tok::kw_catch,
                            tok::kw_new, tok::kw_delete) &&
               (!Left.Previous || Left.Previous->isNot(tok::period))))) ||
Index: lib/Format/UnwrappedLineParser.cpp
===================================================================
--- lib/Format/UnwrappedLineParser.cpp
+++ lib/Format/UnwrappedLineParser.cpp
@@ -1511,6 +1511,8 @@
 void UnwrappedLineParser::parseIfThenElse() {
   assert(FormatTok->Tok.is(tok::kw_if) && "'if' expected");
   nextToken();
+  if (FormatTok->Tok.is(tok::kw_constexpr))
+    nextToken();
   if (FormatTok->Tok.is(tok::l_paren))
     parseParens();
   bool NeedsUnwrappedLine = false;
Index: unittests/Format/FormatTest.cpp
===================================================================
--- unittests/Format/FormatTest.cpp
+++ unittests/Format/FormatTest.cpp
@@ -341,6 +341,18 @@
   verifyFormat("if (true)\n  f();\ng();");
   verifyFormat("if (a)\n  if (b)\n    if (c)\n      g();\nh();");
   verifyFormat("if (a)\n  if (b) {\n    f();\n  }\ng();");
+  verifyFormat("if constexpr (true)\n"
+               "  f();\ng();");
+  verifyFormat("if constexpr (a)\n"
+               "  if constexpr (b)\n"
+               "    if constexpr (c)\n"
+               "      g();\n"
+               "h();");
+  verifyFormat("if constexpr (a)\n"
+               "  if constexpr (b) {\n"
+               "    f();\n"
+               "  }\n"
+               "g();");
 
   FormatStyle AllowsMergedIf = getLLVMStyle();
   AllowsMergedIf.AlignEscapedNewlines = FormatStyle::ENAS_Left;
@@ -423,9 +435,11 @@
   AllowSimpleBracedStatements.AllowShortLoopsOnASingleLine = true;
 
   verifyFormat("if (true) {}", AllowSimpleBracedStatements);
+  verifyFormat("if constexpr (true) {}", AllowSimpleBracedStatements);
   verifyFormat("while (true) {}", AllowSimpleBracedStatements);
   verifyFormat("for (;;) {}", AllowSimpleBracedStatements);
   verifyFormat("if (true) { f(); }", AllowSimpleBracedStatements);
+  verifyFormat("if constexpr (true) { f(); }", AllowSimpleBracedStatements);
   verifyFormat("while (true) { f(); }", AllowSimpleBracedStatements);
   verifyFormat("for (;;) { f(); }", AllowSimpleBracedStatements);
   verifyFormat("if (true) { //\n"
@@ -496,6 +510,19 @@
                "else {\n"
                "  i();\n"
                "}");
+  verifyFormat("if (true)\n"
+               "  if constexpr (true)\n"
+               "    if (true) {\n"
+               "      if constexpr (true)\n"
+               "        f();\n"
+               "    } else {\n"
+               "      g();\n"
+               "    }\n"
+               "  else\n"
+               "    h();\n"
+               "else {\n"
+               "  i();\n"
+               "}");
   verifyFormat("void f() {\n"
                "  if (a) {\n"
                "  } else {\n"
@@ -511,6 +538,12 @@
                "  g();\n"
                "else\n"
                "  h();");
+  verifyFormat("if constexpr (a)\n"
+               "  f();\n"
+               "else if constexpr (b)\n"
+               "  g();\n"
+               "else\n"
+               "  h();");
   verifyFormat("if (a) {\n"
                "  f();\n"
                "}\n"
@@ -528,6 +561,11 @@
                "    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa) {\n"
                "}",
                getLLVMStyleWithColumns(62));
+  verifyFormat("if (a) {\n"
+               "} else if constexpr (\n"
+               "    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa) {\n"
+               "}",
+               getLLVMStyleWithColumns(62));
 }
 
 TEST_F(FormatTest, FormatsForLoop) {
@@ -2371,6 +2409,9 @@
   verifyFormat("if ((aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa ||\n"
                "     bbbbbbbbbbbbbbbbbb) && // aaaaaaaaaaaaaaaa\n"
                "    cccccc) {\n}");
+  verifyFormat("if constexpr ((aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa ||\n"
+               "               bbbbbbbbbbbbbbbbbb) && // aaaaaaaaaaa\n"
+               "              cccccc) {\n}");
   verifyFormat("b = a &&\n"
                "    // Comment\n"
                "    b.c && d;");
@@ -6492,6 +6533,10 @@
                "  if (true) continue;\n"
                "}",
                ShortMergedIf);
+  ShortMergedIf.ColumnLimit = 33;
+  verifyFormat("#define A \\\n"
+               "  if constexpr (true) return 42;",
+               ShortMergedIf);
   ShortMergedIf.ColumnLimit = 29;
   verifyFormat("#define A                   \\\n"
                "  if (aaaaaaaaaa) return 1; \\\n"
@@ -6503,6 +6548,11 @@
                "    return 1;     \\\n"
                "  return 2;",
                ShortMergedIf);
+  verifyFormat("#define A                \\\n"
+               "  if constexpr (aaaaaaa) \\\n"
+               "    return 1;            \\\n"
+               "  return 2;",
+               ShortMergedIf);
 }
 
 TEST_F(FormatTest, FormatStarDependingOnContext) {
@@ -8712,11 +8762,24 @@
                BreakBeforeBraceShortIfs);
   verifyFormat("void f(bool b)\n"
                "{\n"
+               "  if constexpr (b)\n"
+               "  {\n"
+               "    return;\n"
+               "  }\n"
+               "}\n",
+               BreakBeforeBraceShortIfs);
+  verifyFormat("void f(bool b)\n"
+               "{\n"
                "  if (b) return;\n"
                "}\n",
                BreakBeforeBraceShortIfs);
   verifyFormat("void f(bool b)\n"
                "{\n"
+               "  if constexpr (b) return;\n"
+               "}\n",
+               BreakBeforeBraceShortIfs);
+  verifyFormat("void f(bool b)\n"
+               "{\n"
                "  while (b)\n"
                "  {\n"
                "    return;\n"
@@ -9962,6 +10025,11 @@
                "      doo_dah();\n"
                "    })) {\n"
                "}");
+  verifyFormat("if constexpr (blah_blah(whatever, whatever, [] {\n"
+               "                doo_dah();\n"
+               "                doo_dah();\n"
+               "              })) {\n"
+               "}");
   verifyFormat("auto lambda = []() {\n"
                "  int a = 2\n"
                "#if A\n"