[flang] Enforce C1027 (F'2023) (llvm#129617)

The LHS of a pointer assignment statement may not be coindexed.

Author: klausler

flang/lib/Semantics/pointer-assignment.cpp

@@ -154,6 +154,9 @@ bool PointerAssignmentChecker::CheckLeftHandSide(const SomeExpr &lhs) {
   } else if (evaluate::IsAssumedRank(lhs)) {
     Say("The left-hand side of a pointer assignment must not be an assumed-rank dummy argument"_err_en_US);
     return false;
+  } else if (evaluate::ExtractCoarrayRef(lhs)) { // F'2023 C1027
+    Say("The left-hand side of a pointer assignment must not be coindexed"_err_en_US);
+    return false;
   } else {
     return true;
   }
@@ -177,7 +180,7 @@ bool PointerAssignmentChecker::Check(const SomeExpr &rhs) {
     Say("An array section with a vector subscript may not be a pointer target"_err_en_US);
     return false;
   }
-  if (ExtractCoarrayRef(rhs)) { // C1026
+  if (ExtractCoarrayRef(rhs)) { // F'2023 C1029
     Say("A coindexed object may not be a pointer target"_err_en_US);
     return false;
   }

flang/test/Semantics/assign02.f90

@@ -146,14 +146,25 @@ function f2()
     end
   end
 
-  ! C1026 (R1037) A data-target shall not be a coindexed object.
+  ! F'2023 C1029 A data-target shall not be a coindexed object.
   subroutine s10
     real, target, save :: a[*]
     real, pointer :: b
     !ERROR: A coindexed object may not be a pointer target
     b => a[1]
   end
 
+  ! F'2023 C1027 the LHS may not be coindexed
+  subroutine s11
+    type t
+      real, pointer :: p
+    end type
+    type(t), save :: ca[*]
+    real, target :: x
+    !ERROR: The left-hand side of a pointer assignment must not be coindexed
+    ca[1]%p => x
+  end
+
 end
 
 module m2