C++: Fix FPs to cpp/return-stack-allocated-memory

cpp/ql/src/Likely Bugs/Memory Management/ReturnStackAllocatedMemory.ql

@@ -92,6 +92,8 @@ class ReturnStackAllocatedMemoryConfig extends MustFlowConfiguration {
     or
     node2.(PointerOffsetInstruction).getLeftOperand() = node1
   }
+
+  override predicate isBarrier(Instruction n) { n.getResultType() instanceof ErroneousType }
 }
 
 from

cpp/ql/test/query-tests/Likely Bugs/Memory Management/ReturnStackAllocatedMemory/test.cpp

@@ -1,4 +1,4 @@
-// semmle-extractor-options: -std=c++14
+// semmle-extractor-options: -std=c++14 --expect_errors
 class MyClass
 {
 public:
@@ -248,4 +248,9 @@ char* test_strdupa(const char* s) {
 void* test_strndupa(const char* s, size_t size) {
 	char* s2 = strndupa(s, size);
 	return s2; // BAD
-}
+}
+
+UNKNOWN_TYPE test_error_type() {
+	UNKNOWN_TYPE x;
+	return x; // GOOD: Don't report error types
+}