Index: clang/lib/Sema/SemaChecking.cpp
===================================================================
--- clang/lib/Sema/SemaChecking.cpp
+++ clang/lib/Sema/SemaChecking.cpp
@@ -14369,6 +14369,9 @@
 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
@@ -1,19 +1,29 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fsyntax-only -verify=c,expected %s
+// RUN: %clang_cc1 -x c++ -fsyntax-only -verify=cxx,expected %s
 
-struct foo; // expected-note 5 {{forward declaration of 'struct foo'}}
+
+
+struct foo; // c-note 5 {{forward declaration of 'struct foo'}} \
+cxx-note {{forward declaration of 'foo'}} \
+cxx-note {{forward declaration of 'foo'}} \
+cxx-note {{forward declaration of 'foo'}}
 
 void b;  // expected-error {{variable has incomplete type 'void'}}
-struct foo f; // expected-error{{tentative definition has type 'struct foo' that is never completed}}
+struct foo f; // c-error{{tentative definition has type 'struct foo' that is never completed}} \
+cxx-error{{variable has incomplete type 'struct foo'}}
 
 static void c; // expected-error {{variable has incomplete type 'void'}}
-static struct foo g;  // expected-warning {{tentative definition of variable with internal linkage has incomplete non-array type 'struct foo'}} \
-    expected-error{{tentative definition has type 'struct foo' that is never completed}}
+static struct foo g;  // c-warning {{tentative definition of variable with internal linkage has incomplete non-array type 'struct foo'}} \
+c-error{{tentative definition has type 'struct foo' that is never completed}} \
+cxx-error{{variable has incomplete type 'struct foo'}}
 
-extern void d;
+extern void d; // cxx-error {{variable has incomplete type 'void'}}
 extern struct foo e;
 
-int ary[]; // expected-warning {{tentative array definition assumed to have one element}}
-struct foo bary[]; // expected-error {{array has incomplete element type 'struct foo'}}
+int ary[]; // c-warning {{tentative array definition assumed to have one element}} \
+cxx-error {{definition of variable with array type needs an explicit size or an initializer}}
+struct foo bary[]; // c-error {{array has incomplete element type 'struct foo'}} \
+cxx-error {{definition of variable with array type needs an explicit size or an initializer}}
 
 void func(void) {
   int ary[]; // expected-error{{definition of variable with array type needs an explicit size or an initializer}}
@@ -21,11 +31,26 @@
   struct foo f; // expected-error {{variable has incomplete type 'struct foo'}}
 }
 
-int h[]; // expected-warning {{tentative array definition assumed to have one element}}
-int (*i)[] = &h+1; // expected-error {{arithmetic on a pointer to an incomplete type 'int[]'}}
+int h[]; // c-warning {{tentative array definition assumed to have one element}} \
+cxx-error {{definition of variable with array type needs an explicit size or an initializer}}
+int (*i)[] = &h+1; // c-error {{arithmetic on a pointer to an incomplete type 'int[]'}}
 
-struct bar j = {1}; // expected-error {{variable has incomplete type 'struct bar'}} \
-    expected-note {{forward declaration of 'struct bar'}}
-struct bar k;
+                               struct bar j = {1}; // expected-error {{variable has incomplete type 'struct bar'}} \
+c-note {{forward declaration of 'struct bar'}} \
+cxx-note {{forward declaration of 'bar'}} \
+cxx-note {{forward declaration of 'bar'}}
+
+struct bar k; // cxx-error {{variable has incomplete type 'struct bar'}}
 struct bar { int a; };
 
+struct x y; //c-note {{forward declaration of 'struct x'}} \
+c-note {{forward declaration of 'struct x'}} \
+cxx-error {{variable has incomplete type 'struct x'}} \
+cxx-note {{forward declaration of 'x'}}
+void foo() {
+  (void)(1 ? y : y); // c-error {{incomplete type 'struct x' where a complete type is required}} \
+c-error {{incomplete type 'struct x' where a complete type is required}}
+}
+struct x{
+  int a;
+};