[flang] Support BIND(C) variable scope check

[peixin]

Summary

As Fortran 2018 C819, a variable with the BIND attribute shall be declared
in the specification part of a module. Add the support for this check.

[klausler]

Your change will still allow BIND(C) variables to be declared in submodules, and I don't think that is allowed.

[peixin]

Thanks @klausler for the comment. Addressed it.

[klausler]

flang/lib/Semantics/check-declarations.cpp

107

Rather than making the name of this function longer (because it does more now), consider making it shorter, like "CheckBindC", since further checks may be added to it in the future.

1874

Consider adding a CHECK() here to ensure that the symbol is BIND(C). Or test for BIND(C) and return if it is not, in which case you can continue to call it unconditionally. The second option seems safer.

[peixin]

Address the comments:

1. Rename to CheckBindC.

2. Call it unconditionally and return if it does not have BIND(C) attr.

flang/lib/Semantics/check-declarations.cpp

private:
   void SayWithDeclaration(const Symbol &symbol, A &&...x) {
     if (parser::Message * msg{messages_.Say(std::forward<A>(x)...)}) {
       if (messages_.at().begin() != symbol.name().begin()) {
         evaluate::AttachDeclaration(*msg, symbol);
       }
     }
   }
   bool IsResultOkToDiffer(const FunctionResult &);
-  void CheckBindCName(const Symbol &);
+  void CheckBindC(const Symbol &);
   // Check functions for defined I/O procedures
   void CheckDefinedIoProc(
       const Symbol &, const GenericDetails &, GenericKind::DefinedIo);
   bool CheckDioDummyIsData(const Symbol &, const Symbol *, std::size_t);
   void CheckDioDummyIsDerived(const Symbol &, const Symbol &,
       GenericKind::DefinedIo ioKind, const Symbol &);
   void CheckDioDummyIsDefaultInteger(const Symbol &, const Symbol &);
   void CheckDioDummyIsScalar(const Symbol &, const Symbol &);
common::visit(
           [&](const DerivedTypeDetails &x) { CheckDerivedType(symbol, x); },
           [&](const GenericDetails &x) { CheckGeneric(symbol, x); },
           [](const auto &) {},
       },
       symbol.details());
   if (symbol.attrs().test(Attr::VOLATILE)) {
     CheckVolatile(symbol, derived);
   }
-  CheckBindCName(symbol);
+  CheckBindC(symbol);
   if (isDone) {
     return; // following checks do not apply
   }
   if (IsPointer(symbol)) {
     CheckPointer(symbol);
   }
   if (InPure()) {
     if (IsSaved(symbol)) {
if ((subp && !subp->isInterface() &&
       symbol.has<ObjectEntityDetails>()) {
     // Symbol defines data or entry point
     return symbol.GetBindName();
   } else {
     return nullptr;
   }
 }
 
-// Check that BIND(C) names are distinct
-void CheckHelper::CheckBindCName(const Symbol &symbol) {
+// Check that BIND(C) names are distinct and BIND(C) variable declared in module
+void CheckHelper::CheckBindC(const Symbol &symbol) {
+  if (!symbol.attrs().test(Attr::BIND_C)) {
+    return;
+  }
+  if (symbol.has<ObjectEntityDetails>() && !symbol.owner().IsModule()) {
+    messages_.Say(symbol.name(),
+        "A variable with BIND(C) attribute may only appear in the specification part of a module"_err_en_US);
+  }
   if (const std::string * name{DefinesBindCName(symbol)}) {
     auto pair{bindC_.emplace(*name, symbol)};
     if (!pair.second) {
       const Symbol &other{*pair.first->second};
       if (DefinesBindCName(other) && !context_.HasError(other)) {
         if (auto *msg{messages_.Say(
                 "Two symbols have the same BIND(C) name '%s'"_err_en_US,
                 *name)}) {

flang/test/Semantics/resolve113.f90

+! RUN: %python %S/test_errors.py %s %flang_fc1
+
+module m
+interface
+  module subroutine dump()
+  end subroutine
+end interface
+  integer, bind(c, name="a") :: x1
+  integer, bind(c) :: x2
+end
+
+subroutine sub()
+  !ERROR: A variable with BIND(C) attribute may only appear in the specification part of a module
+  integer, bind(c, name="b") :: x3
+  !ERROR: A variable with BIND(C) attribute may only appear in the specification part of a module
+  integer, bind(c) :: x4
+end
+
+program main
+  !ERROR: A variable with BIND(C) attribute may only appear in the specification part of a module
+  integer, bind(c, name="c") :: x5
+  !ERROR: A variable with BIND(C) attribute may only appear in the specification part of a module
+  integer, bind(c) :: x6
+end
+
+submodule(m) m2
+  !ERROR: A variable with BIND(C) attribute may only appear in the specification part of a module
+  integer, bind(c, name="d") :: x7
+  !ERROR: A variable with BIND(C) attribute may only appear in the specification part of a module
+  integer, bind(c) :: x8
+contains
+  module procedure dump
+  end procedure
+end