Index: lib/Parse/ParseExprCXX.cpp
===================================================================
--- lib/Parse/ParseExprCXX.cpp
+++ lib/Parse/ParseExprCXX.cpp
@@ -1740,11 +1740,25 @@
   ParsedAttributesWithRange attrs(AttrFactory);
   MaybeParseCXX11Attributes(attrs);
 
+  const auto WarnOnInit = [this, &CK] {
+    Diag(Tok.getLocation(), getLangOpts().CPlusPlus1z
+                                ? diag::warn_cxx14_compat_init_statement
+                                : diag::ext_init_statement)
+        << (CK == Sema::ConditionKind::Switch);
+  };
+
   // Determine what kind of thing we have.
   switch (isCXXConditionDeclarationOrInitStatement(InitStmt)) {
   case ConditionOrInitStatement::Expression: {
     ProhibitAttributes(attrs);
 
+    // We can have an empty expression here.
+    //   if (; true);
+    if (InitStmt && TryConsumeToken(tok::semi)) {
+      WarnOnInit();
+      return ParseCXXCondition(nullptr, Loc, CK);
+    }
+
     // Parse the expression.
     ExprResult Expr = ParseExpression(); // expression
     if (Expr.isInvalid())
@@ -1760,10 +1774,7 @@
   }
 
   case ConditionOrInitStatement::InitStmtDecl: {
-    Diag(Tok.getLocation(), getLangOpts().CPlusPlus1z
-                                ? diag::warn_cxx14_compat_init_statement
-                                : diag::ext_init_statement)
-        << (CK == Sema::ConditionKind::Switch);
+    WarnOnInit();
     SourceLocation DeclStart = Tok.getLocation(), DeclEnd;
     DeclGroupPtrTy DG = ParseSimpleDeclaration(
         Declarator::InitStmtContext, DeclEnd, attrs, /*RequireSemi=*/true);
Index: test/CXX/stmt.stmt/stmt.select/p3.cpp
===================================================================
--- test/CXX/stmt.stmt/stmt.select/p3.cpp
+++ test/CXX/stmt.stmt/stmt.select/p3.cpp
@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++1z -verify %s -DCXX17
 
 int f();
 
@@ -10,10 +11,52 @@
   }
 }
 
-
 void h() {
   if (int x = f()) // expected-note 2{{previous definition}}
     int x; // expected-error{{redefinition of 'x'}}
   else
     int x; // expected-error{{redefinition of 'x'}}
 }
+
+#ifdef CXX17
+int ifInitStatement() {
+  if (int I = 0; ++I == 1)
+    return I;
+
+  int Var = 0;
+  if (Var + Var; Var == 0)
+    return Var;
+
+  if (; true)
+    return 1;
+}
+
+int switchInitStatement() {
+  switch (int I = 1; I) {
+  case 1:
+    return I;
+  }
+
+  int Var = 0;
+  switch (Var + Var; Var) {
+  case 0:
+    return Var;
+  }
+
+  switch (; 0) {
+  case 0:
+    return 0;
+  }
+}
+#endif
+
+int whileInitStatement() {
+  while (int I = 10; I--); // expected-error{{expected ')'}}
+  // expected-note@-1{{to match this '('}}
+  // expected-error@-2{{use of undeclared identifier 'I'}}
+
+  int Var = 10;
+  while (; Var--); // expected-error{{expected expression}}
+  // expected-error@-1{{extraneous ')' before ';'}}
+}
+