[flang] Allow NULL() actual argument for optional dummy procedure

[klausler]

Summary

A disassociated procedure pointer is allowed to be passed as an absent
actual argument that corresponds to an optional dummy procedure,
but not NULL(); accept that case as well.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Semantics/check-call.cpp

if (auto argChars{characteristics::DummyArgument::FromActual(
             }
           }
         } else {
           messages.Say(
               "Actual argument associated with procedure %s is not a procedure"_err_en_US,
               dummyName);
         }
       } else if (IsNullPointer(*expr)) {
-        if (!dummyIsPointer) {
+        if (!dummyIsPointer &&
+            !dummy.attrs.test(
+                characteristics::DummyProcedure::Attr::Optional)) {
           messages.Say(
               "Actual argument associated with procedure %s is a null pointer"_err_en_US,
               dummyName);
         }
       } else {
         messages.Say(
             "Actual argument associated with procedure %s is typeless"_err_en_US,
             dummyName);

flang/test/Semantics/call02.f90

interface
     subroutine subr(dummy)
       procedure(sin) :: dummy
     end subroutine
     subroutine badsubr(dummy)
       import :: elem
       !ERROR: A dummy procedure may not be ELEMENTAL
       procedure(elem) :: dummy
     end subroutine
+    subroutine optionalsubr(dummy)
+      procedure(sin), optional :: dummy
+    end subroutine
+    subroutine ptrsubr(dummy)
+      procedure(sin), pointer, intent(in) :: dummy
+    end subroutine
   end interface
   intrinsic :: cos
   call subr(cos) ! not an error
   !ERROR: Non-intrinsic ELEMENTAL procedure 'elem' may not be passed as an actual argument
   call subr(elem) ! C1533
   !ERROR: Actual argument associated with procedure dummy argument 'dummy=' is a null pointer
   call subr(null())
+  call optionalsubr(null()) ! ok
+  call ptrsubr(null()) ! ok
   !ERROR: Actual argument associated with procedure dummy argument 'dummy=' is typeless
   call subr(B"1010")
 end subroutine
 
 subroutine s02
   !ERROR: Non-intrinsic ELEMENTAL procedure 'elem' may not be passed as an actual argument
   call sub(elem)
  contains