A18-5-8: address fp report #20, add return expression flow for NRVO elision

Author: knewbury01

change_notes/2024-05-22-fix-fp-rule-A18-5-8.md

@@ -0,0 +1,2 @@
+- `A18-5-8` - `UnnecessaryUseOfDynamicStorage.ql`:
+  - Address FP reported in #20. Add model of flow from MakeSharedOrUnique to return expression to capture copy/move elision case NRVO.

cpp/autosar/src/rules/A18-5-8/UnnecessaryUseOfDynamicStorage.ql

@@ -53,6 +53,9 @@ class MakeSharedOrUnique extends FunctionCall, CandidateFunctionLocalHeapAllocat
     // This includes the case where a result of `make_shared` or `make_unique` is return by a function
     // because the compiler will call the appropriate constructor.
     not exists(FunctionCall fc | DataFlow::localExprFlow(this, fc.getAnArgument())) and
+    // The flow to a return statement is explicitly modelled for the case where
+    // the copy/move constructor is elided and therefore there is no actual function call in the database
+    not exists(ReturnStmt ret | DataFlow::localExprFlow(this, ret.getExpr())) and
     // Not assigned to a field
     not exists(Field f | DataFlow::localExprFlow(this, f.getAnAssignedValue()))
   }

cpp/autosar/test/rules/A18-5-8/test.cpp

@@ -68,4 +68,13 @@ StructA *test_failure() {
     a = nullptr;
   }
   return a;
+}
+
+#include <string>
+std::unique_ptr<StructA>
+test_for_fp_reported_in_20(const std::string &s) noexcept {
+  // make_unique performs heap allocation
+  // but this outlives the function due to copy elision
+  // (specifically NRVO)
+  return std::make_unique<StructA>(s); // COMPLIANT
 }