Index: clang/lib/Sema/SemaChecking.cpp
===================================================================
--- clang/lib/Sema/SemaChecking.cpp
+++ clang/lib/Sema/SemaChecking.cpp
@@ -14369,6 +14369,10 @@
 static void CheckConditionalOperand(Sema &S, Expr *E, QualType T,
                                     SourceLocation CC, bool &ICContext) {
   E = E->IgnoreParenImpCasts();
+  // Diagnose incomplete type for second or third operand in C.
+  if (!S.getLangOpts().CPlusPlus && E->getType()->isRecordType()) {
+    S.RequireCompleteExprType(E, diag::err_incomplete_type);
+  }
 
   if (auto *CO = dyn_cast<AbstractConditionalOperator>(E))
     return CheckConditionalOperator(S, CO, CC, T);
Index: clang/test/Sema/incomplete-decl.c
===================================================================
--- clang/test/Sema/incomplete-decl.c
+++ clang/test/Sema/incomplete-decl.c
@@ -1,5 +1,15 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s
-
+// RUN: %clang_cc1 -fsyntax-only -x c++ -verify %s
+#ifdef __cplusplus
+struct x y; // expected-error {{variable has incomplete type 'struct x'}} \
+      expected-note {{forward declaration of 'x'}} 
+void foo() {
+  (void)(1 ? y : y); 
+}
+struct x{
+  int a;
+};
+#else
 struct foo; // expected-note 5 {{forward declaration of 'struct foo'}}
 
 void b;  // expected-error {{variable has incomplete type 'void'}}
@@ -29,3 +39,14 @@
 struct bar k;
 struct bar { int a; };
 
+struct x y;  //expected-note {{forward declaration of 'struct x'}}\
+//expected-note {{forward declaration of 'struct x'}}
+void foo() {
+  (void)(1 ? y : y); // expected-error {{incomplete type 'struct x' where a complete type is required}} \
+      expected-error {{incomplete type 'struct x' where a complete type is required}}
+}
+struct x{
+  int a;
+};
+
+#endif