[flang] Better error message for NULL() actual argument for dummy allocatable

[klausler]

Summary

f18 intentionally does not support the spottily-implemented language extension
in which one can pass NULL() for an allocatable dummy argument. This is perhaps
a sanctioned side effect in other compilers of the fact that they pass distinct
"base address" and "descriptor address" physical arguments.

Make the error message in this case more specific to the circumstances, and
add a note to Extensions.md to clarify that this behavior is intended.

(We could, with some effort in lowering, support passing NULL for an INTENT(IN)
allocatable dummy, but let's see whether such nonconforming usage appears
in a real application before spending any more time on it.)

[jeanPerier]

Thanks

flang/docs/Extensions.md

   PGI converts the arguments while Intel and XLF replace the specific by the related generic.
 * VMS listing control directives (`%LIST`, `%NOLIST`, `%EJECT`)
 * Continuation lines on `INCLUDE` lines
+* `NULL()` actual argument corresponding to an `ALLOCATABLE` dummy data object
 
 ## Preprocessing behavior
 

flang/lib/Semantics/check-call.cpp

                                        DummyDataObject::Attr::Optional)) &&
                     evaluate::IsNullPointer(*expr)) {
                   // ok, FOO(NULL())
+                } 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,
+                      expr->AsFortran(), dummyName);
                 } else {
                   messages.Say(
                       "Actual argument '%s' associated with %s is not a variable or typed expression"_err_en_US,

flang/test/Semantics/call27.f90

+! RUN: %python %S/test_errors.py %s %flang_fc1
+! Catch NULL() actual argement association with allocatable dummy argument
+program test
+  !ERROR: Null actual argument 'NULL()' may not be associated with allocatable dummy argument 'a='
+  call foo1(null())
+  !ERROR: Null actual argument 'NULL()' may not be associated with allocatable dummy argument 'a='
+  call foo2(null()) ! perhaps permissible later on user request
+  call foo3(null()) ! ok
+ contains
+  subroutine foo1(a)
+    real, allocatable :: a
+  end subroutine
+  subroutine foo2(a)
+    real, allocatable, intent(in) :: a
+  end subroutine
+  subroutine foo3(a)
+    real, allocatable, optional :: a
+  end subroutine
+end