[flang] Further work on NULL(MOLD=allocatable) (#129345)

Refine handling of NULL(...) in semantics to properly distinguish
NULL(), NULL(objectPointer), NULL(procPointer), and NULL(allocatable)
from each other in relevant contexts.

Add IsNullAllocatable() and IsNullPointerOrAllocatable() utility
functions. IsNullAllocatable() is true only for NULL(allocatable); it is
false for a bare NULL(), which can be detected independently with
IsBareNullPointer().

IsNullPointer() now returns false for NULL(allocatable).

ALLOCATED(NULL(allocatable)) now works, and folds to .FALSE.

These utilities were modified to accept const pointer arguments rather
than const references; I usually prefer this style when the result
should clearly be false for a null argument (in the C sense), and it
helped me find all of their use sites in the code.

1 files changed, 1 insertions, 1 deletions

diff --git a/flang/lib/Semantics/pointer-assignment.cpp b/flang/lib/Semantics/pointer-assignment.cpp
index 7f4548c..654c018 100644
--- a/flang/lib/Semantics/pointer-assignment.cpp
+++ b/flang/lib/Semantics/pointer-assignment.cpp
@@ -184,7 +184,7 @@ bool PointerAssignmentChecker::Check(const SomeExpr &rhs) {
   if (!common::visit([&](const auto &x) { return Check(x); }, rhs.u)) {
     return false;
   }
-  if (IsNullPointer(rhs)) {
+  if (IsNullPointer(&rhs)) {
     return true;
   }
   if (lhs_ && IsProcedure(*lhs_)) {