Index: clang-tools-extra/trunk/clang-tidy/utils/TypeTraits.cpp
===================================================================
--- clang-tools-extra/trunk/clang-tidy/utils/TypeTraits.cpp
+++ clang-tools-extra/trunk/clang-tidy/utils/TypeTraits.cpp
@@ -41,7 +41,7 @@
 
 llvm::Optional<bool> isExpensiveToCopy(QualType Type,
                                        const ASTContext &Context) {
-  if (Type->isDependentType())
+  if (Type->isDependentType() || Type->isIncompleteType())
     return llvm::None;
   return !Type.isTriviallyCopyableType(Context) &&
          !classHasTrivialCopyAndDestroy(Type) &&
Index: clang-tools-extra/trunk/test/clang-tidy/performance-unnecessary-value-param.cpp
===================================================================
--- clang-tools-extra/trunk/test/clang-tidy/performance-unnecessary-value-param.cpp
+++ clang-tools-extra/trunk/test/clang-tidy/performance-unnecessary-value-param.cpp
@@ -1,4 +1,4 @@
-// RUN: %check_clang_tidy %s performance-unnecessary-value-param %t
+// RUN: %check_clang_tidy %s performance-unnecessary-value-param %t -- -fix-errors -- --std=c++11
 
 // CHECK-FIXES: #include <utility>
 
@@ -237,3 +237,10 @@
   ExpensiveToCopyType B;
   B = A;
 }
+
+// Ensure that incomplete types result in an error from the frontend and not a
+// clang-tidy diagnostic about IncompleteType being expensive to copy.
+struct IncompleteType;
+void NegativeForIncompleteType(IncompleteType I) {
+  // CHECK-MESSAGES: [[@LINE-1]]:47: error: variable has incomplete type 'IncompleteType' [clang-diagnostic-error]
+}