[flang] Enforce C1529 as a warning, C919 as an error

[klausler]

Summary

Constraint C1529 requires that the base object of a type-bound procedure
reference be a scalar if the TBP has the NOPASS attribute. Most
compilers do not enforce this constraint and it does not appear to
have any implementation justification, so emit portability warning.

On the other hand, we fail to enforce C919 for references to
procedure pointer components, whose base objects must of course
be scalars in order to avoid ambiguity and empty arrays, whether
NOPASS is present or not.

[clementval]

LGTM

flang/docs/Extensions.md

* When a type-bound procedure declaration statement has neither interface
   if a binding has renaming with "=> proc".
   The colons are not necessary for an unambiguous parse, C768
   notwithstanding.
 * A type-bound procedure binding can be passed as an actual
   argument corresponding to a dummy procedure and can be used as
   the target of a procedure pointer assignment statement.
 * An explicit `INTERFACE` can declare the interface of a
   procedure pointer even if it is not a dummy argument.
+* A `NOPASS` type-bound procedure binding is required by C1529
+  to apply only to a scalar data-ref, but most compilers don't
+  enforce it and the constraint is not necessary for a correct
+  implementation.
 
 ### Extensions supported when enabled by options
 
 * C-style backslash escape sequences in quoted CHARACTER literals
   (but not Hollerith) [-fbackslash]
 * Logical abbreviations `.T.`, `.F.`, `.N.`, `.A.`, `.O.`, and `.X.`
   [-flogical-abbreviations]
 * `.XOR.` as a synonym for `.NEQV.` [-fxor-operator]

flang/lib/Semantics/expression.cpp

if (const Symbol *sym{sc.component.symbol}) {
                 *sc.component.symbol, pair.second, isSubroutine);
             return std::nullopt;
           }
         }
         std::optional<DataRef> dataRef{ExtractDataRef(std::move(*dtExpr))};
         if (dataRef && !CheckDataRef(*dataRef)) {
           return std::nullopt;
         }
-        if (dataRef && dataRef->Rank() > 0 && sym->attrs().test(semantics::Attr::NOPASS)) {
+        if (dataRef && dataRef->Rank() > 0) {
+          if (sym->has<semantics::ProcBindingDetails>() &&
+              sym->attrs().test(semantics::Attr::NOPASS)) {
             // C1529 seems unnecessary and most compilers don't enforce it.
+            AttachDeclaration(
+                Say(sc.component.source,
+                    "Base of NOPASS type-bound procedure reference should be scalar"_port_en_US),
+                *sym);
+          } else if (IsProcedurePointer(*sym)) { // C919
             Say(sc.component.source,
-            "Base of procedure component reference should be scalar when NOPASS component or binding '%s' is referenced"_port_en_US, sc.component.source);
+                "Base of procedure component reference must be scalar"_err_en_US);
+          }
         }
         if (const Symbol *resolution{
                 GetBindingResolution(dtExpr->GetType(), *sym)}) {
           AddPassArg(arguments, std::move(*dtExpr), *sym, false);
           return CalleeAndArguments{
               ProcedureDesignator{*resolution}, std::move(arguments)};
         } else if (dataRef.has_value()) {
           if (sym->attrs().test(semantics::Attr::NOPASS)) {

flang/test/Semantics/bindings01.f90

module m7
   end type
 contains
   subroutine b(*, passArg)
     class(t) :: passArg
     return 1
   end subroutine
 end module m7
 
+module m8 ! C1529 - warning only
+  type t
+    procedure(mysubr), pointer, nopass :: pp
+   contains
+    procedure, nopass :: tbp => mysubr
+  end type
+ contains
+  subroutine mysubr
+  end subroutine
+  subroutine test
+    type(t) a(2)
+    !PORTABILITY: Base of NOPASS type-bound procedure reference should be scalar
+    call a%tbp
+    !ERROR: Base of procedure component reference must be scalar
+    call a%pp
+  end subroutine
+end module
+
 program test
   use m1
   type,extends(t) :: t2
   end type
   type(t2) a
   call a%tbp
 end program