[flang][runtime] Initialize LHS temporary in AssignTemporary.

If LHS is of derived type that needs initialization, then it must be
initialized before doing the assignment. Otherwise, the assignment
might behave incorrectly based on uninitialized components that are
descriptors themselves.

Differential Revision: https://reviews.llvm.org/D149681

flang/runtime/assign.cpp

@@ -546,6 +546,17 @@ void RTNAME(Assign)(Descriptor &to, const Descriptor &from,
 void RTNAME(AssignTemporary)(Descriptor &to, const Descriptor &from,
     const char *sourceFile, int sourceLine) {
   Terminator terminator{sourceFile, sourceLine};
+  // Initialize the "to" if it is of derived type that needs initialization.
+  if (const DescriptorAddendum * addendum{to.Addendum()}) {
+    if (const auto *derived{addendum->derivedType()}) {
+      if (!derived->noInitializationNeeded()) {
+        if (ReturnError(terminator, Initialize(to, *derived, terminator)) !=
+            StatOk) {
+          return;
+        }
+      }
+    }
+  }
   Assign(to, from, terminator, PolymorphicLHS);
 }