[flang] Enforce F'2023 C1520 correctly (#82842)

When a procedure declaration statement has a binding label, it must
declare no more than one procedure.

Fixes #82528.

flang/lib/Semantics/resolve-names.cpp

@@ -1118,7 +1118,6 @@ class DeclarationVisitor : public ArraySpecVisitor,
   // Set when walking DATA & array constructor implied DO loop bounds
   // to warn about use of the implied DO intex therein.
   std::optional<SourceName> checkIndexUseInOwnBounds_;
-  bool hasBindCName_{false};
   bool isVectorType_{false};
   UnorderedSymbolSet mustBeScalar_;
 
@@ -5589,7 +5588,10 @@ bool DeclarationVisitor::Pre(const parser::ProcedureDeclarationStmt &x) {
   for (const parser::ProcAttrSpec &procAttr : procAttrSpec) {
     if (auto *bindC{std::get_if<parser::LanguageBindingSpec>(&procAttr.u)}) {
       if (bindC->v.has_value()) {
-        hasBindCName_ = true;
+        if (std::get<std::list<parser::ProcDecl>>(x.t).size() > 1) {
+          Say(context().location().value(),
+              "A procedure declaration statement with a binding name may not declare multiple procedures"_err_en_US);
+        }
         break;
       }
     }
@@ -5598,7 +5600,6 @@ bool DeclarationVisitor::Pre(const parser::ProcedureDeclarationStmt &x) {
 }
 void DeclarationVisitor::Post(const parser::ProcedureDeclarationStmt &) {
   interfaceName_ = nullptr;
-  hasBindCName_ = false;
   EndDecl();
 }
 bool DeclarationVisitor::Pre(const parser::DataComponentDefStmt &x) {

flang/test/Semantics/bind-c04.f90

@@ -19,7 +19,7 @@ subroutine aproc2() bind(c) ! ok
     end
   end interface
 
-  !Acceptable (as an extension)
+  !ERROR: A procedure declaration statement with a binding name may not declare multiple procedures
   procedure(proc), bind(c, name="aaa") :: pc1, pc2
 
   !ERROR: A procedure pointer may not have a BIND attribute with a name