Index: clang-tools-extra/trunk/clang-tidy/bugprone/UndefinedMemoryManipulationCheck.cpp
===================================================================
--- clang-tools-extra/trunk/clang-tidy/bugprone/UndefinedMemoryManipulationCheck.cpp
+++ clang-tools-extra/trunk/clang-tidy/bugprone/UndefinedMemoryManipulationCheck.cpp
@@ -19,7 +19,8 @@
 
 namespace {
 AST_MATCHER(CXXRecordDecl, isNotTriviallyCopyable) {
-  return !Node.isTriviallyCopyable();
+  // For incomplete types, assume they are TriviallyCopyable.
+  return Node.hasDefinition() ? !Node.isTriviallyCopyable() : false;
 }
 } // namespace
 
Index: clang-tools-extra/trunk/test/clang-tidy/bugprone-undefined-memory-manipulation.cpp
===================================================================
--- clang-tools-extra/trunk/test/clang-tidy/bugprone-undefined-memory-manipulation.cpp
+++ clang-tools-extra/trunk/test/clang-tidy/bugprone-undefined-memory-manipulation.cpp
@@ -52,6 +52,13 @@
   int vb;
 };
 
+// Incomplete type, assume it is TriviallyCopyable.
+struct NoDef;
+
+void f(NoDef *s) {
+  memset(s, 0, 5);
+}
+
 template <typename T>
 void memset_temp(T *b) {
   memset(b, 0, sizeof(T));