[flang] Enforce restrictions on intrinsic assignment

[klausler]

Summary

When the left-hand side of an intrinsic assignment statement is
polymorphic, the LHS must be a whole allocatable variable or
component and may not be a coarray (10.2.2.1p1(1)).

[klausler]

The discrepancy in the results of the call28 test in the CI builds will clear up after a rebase when another patch is merged.

flang/lib/Semantics/expression.cpp

if (!x.typedAssignment) {
     analyzer.Analyze(std::get<parser::Expr>(x.t));
     std::optional<Assignment> assignment;
     if (!analyzer.fatalErrors()) {
       auto restorer{GetContextualMessages().SetLocation(variable.GetSource())};
       std::optional<ProcedureRef> procRef{analyzer.TryDefinedAssignment()};
       if (!procRef) {
         analyzer.CheckForNullPointer(
             "in a non-pointer intrinsic assignment statement");
+        const Expr<SomeType> &lhs{analyzer.GetExpr(0)};
+        if (auto dyType{lhs.GetType()};
+            dyType && dyType->IsPolymorphic()) { // 10.2.1.2p1(1)
+          const Symbol *lastWhole0{UnwrapWholeSymbolOrComponentDataRef(lhs)};
+          const Symbol *lastWhole{
+              lastWhole0 ? &lastWhole0->GetUltimate() : nullptr};
+          if (!lastWhole || !IsAllocatable(*lastWhole)) {
+            Say("Left-hand side of assignment may not be polymorphic unless assignment is to an entire allocatable"_err_en_US);
+          } else if (evaluate::IsCoarray(*lastWhole)) {
+            Say("Left-hand side of assignment may not be polymorphic if it is a coarray"_err_en_US);
+          }
+        }
       }
       assignment.emplace(analyzer.MoveExpr(0), analyzer.MoveExpr(1));
       if (procRef) {
         assignment->u = std::move(*procRef);
       }
     }
     x.typedAssignment.Reset(new GenericAssignmentWrapper{std::move(assignment)},
         GenericAssignmentWrapper::Deleter);

flang/test/Semantics/assign11.f90

+! RUN: %python %S/test_errors.py %s %flang_fc1
+! 10.2.1.2p1(1)
+program test
+  class(*), allocatable :: pa
+  class(*), pointer :: pp
+  class(*), allocatable :: pac[:]
+  pa = 1 ! ok
+  !ERROR: Left-hand side of assignment may not be polymorphic unless assignment is to an entire allocatable
+  pp = 1
+  !ERROR: Left-hand side of assignment may not be polymorphic if it is a coarray
+  pac = 1
+end

flang/test/Semantics/call28.f90

 ! RUN: %python %S/test_errors.py %s %flang_fc1
 
 module m1
   type :: t
   end type
  contains
   pure subroutine s1(x)
     class(t), intent(in out) :: x
     call s2(x)
     call s3(x)
   end subroutine
   pure subroutine s2(x)
     class(t), intent(in out) :: x
+    !ERROR: Left-hand side of assignment may not be polymorphic unless assignment is to an entire allocatable
     !ERROR: Left-hand side of assignment is not definable
     !BECAUSE: 'x' is polymorphic in a pure subprogram
     x = t()
   end subroutine
   pure subroutine s3(x)
     !ERROR: An INTENT(OUT) dummy argument of a pure subroutine may not be polymorphic
     class(t), intent(out) :: x
   end subroutine
 end module