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();
+  if (!S.getLangOpts().CPlusPlus && E->getType()->isRecordType()) {
+    S.RequireCompleteType(E->getBeginLoc(), E->getType(),
+                          diag::err_incomplete_type);
+  }
 
   if (auto *CO = dyn_cast<AbstractConditionalOperator>(E))
     return CheckConditionalOperator(S, CO, CC, T);