[flang] Ensure that NULL(without MOLD=) not passed to dummy argument with assumed type parameters

[klausler]

Summary

A dummy argument with an assumed (*) character length or derived type parameter
value specification needs to be associated with an actual argument that can
supply a value for it, so make sure that a NULL without a MOLD= is not being
passed.

[clementval]

LGTM

flang/lib/Semantics/check-call.cpp

common::visit(
                   CheckExplicitDataArg(object, dummyName, *expr, *type,
                       isElemental, context, foldingContext, scope, intrinsic,
                       allowActualArgumentConversions, extentErrors, proc);
                 } else if (object.type.type().IsTypelessIntrinsicArgument() &&
                     IsBOZLiteral(*expr)) {
                   // ok
                 } else if (object.type.type().IsTypelessIntrinsicArgument() &&
                     evaluate::IsNullObjectPointer(*expr)) {
-                  // ok, ASSOCIATED(NULL())
+                  // ok, ASSOCIATED(NULL(without MOLD=))
                 } else if ((object.attrs.test(characteristics::DummyDataObject::
                                     Attr::Pointer) ||
                                object.attrs.test(characteristics::
                                        DummyDataObject::Attr::Optional)) &&
                     evaluate::IsNullObjectPointer(*expr)) {
-                  // ok, FOO(NULL())
+                  // FOO(NULL(without MOLD=))
+                  if (object.type.type().IsAssumedLengthCharacter()) {
+                    messages.Say(
+                        "Actual argument associated with %s is a NULL() pointer without a MOLD= to provide a character length"_err_en_US,
+                        dummyName);
+                  } else if (const DerivedTypeSpec *
+                      derived{GetDerivedTypeSpec(object.type.type())}) {
+                    for (const auto &[pName, pValue] : derived->parameters()) {
+                      if (pValue.isAssumed()) {
+                        messages.Say(
+                            "Actual argument associated with %s is a NULL() pointer without a MOLD= to provide a value for the assumed type parameter '%s'"_err_en_US,
+                            dummyName, pName.ToString());
+                        break;
+                      }
+                    }
+                  }
                 } else if (object.attrs.test(characteristics::DummyDataObject::
                                    Attr::Allocatable) &&
                     evaluate::IsNullPointer(*expr)) {
                   // Unsupported extension that more or less naturally falls
                   // out of other Fortran implementations that pass separate
                   // base address and descriptor address physical arguments
                   messages.Say(
                       "Null actual argument '%s' may not be associated with allocatable %s"_err_en_US,

flang/test/Semantics/null01.f90

subroutine test
   print *, transfer(null(rp0),ip0)
   !ERROR: NULL() may not be used as an expression in this context
   select case(null(ip0))
   end select
   !ERROR: NULL() may not be used as an expression in this context
   if (null(lp)) then
   end if
 end subroutine test
+
+module m
+  type :: pdt(n)
+    integer, len :: n
+  end type
+ contains
+  subroutine s1(x)
+    character(*), pointer, intent(in) :: x
+  end
+  subroutine s2(x)
+    type(pdt(*)), pointer, intent(in) :: x
+  end
+  subroutine test
+    !ERROR: Actual argument associated with dummy argument 'x=' is a NULL() pointer without a MOLD= to provide a character length
+    call s1(null())
+    !ERROR: Actual argument associated with dummy argument 'x=' is a NULL() pointer without a MOLD= to provide a value for the assumed type parameter 'n'
+    call s2(null())
+  end
+end