[flang] Disallow noninteroperable dummy procedures from interoperable procedures

[klausler]

Summary

A BIND(C) interoperable procedure must have only interoperable dummy procedures.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Semantics/check-declarations.cpp

if (const auto *type{symbol.GetType()}) {
         context_.SetError(symbol);
       }
     }
     if (IsOptional(symbol) && !symbol.attrs().test(Attr::VALUE)) {
       WarnIfNotInModuleFile(symbol.name(),
           "An interoperable procedure with an OPTIONAL dummy argument might not be portable"_port_en_US);
     }
   } else if (const auto *proc{symbol.detailsIf<ProcEntityDetails>()}) {
-    if (!proc->isDummy() &&
-        (!proc->procInterface() ||
-            !proc->procInterface()->attrs().test(Attr::BIND_C))) {
+    if (!proc->procInterface() ||
+        !proc->procInterface()->attrs().test(Attr::BIND_C)) {
+      if (proc->isDummy()) {
+        messages_.Say(symbol.name(),
+            "A dummy procedure to an interoperable procedure must also be interoperable"_err_en_US);
+        context_.SetError(symbol);
+      } else {
         messages_.Say(symbol.name(),
             "An interface name with BIND attribute must be specified if the BIND attribute is specified in a procedure declaration statement"_err_en_US);
         context_.SetError(symbol);
       }
+    }
   } else if (const auto *subp{symbol.detailsIf<SubprogramDetails>()}) {
     for (const Symbol *dummy : subp->dummyArgs()) {
       if (dummy) {
         CheckBindC(*dummy);
       } else {
         messages_.Say(symbol.name(),
             "A subprogram interface with the BIND attribute may not have an alternate return argument"_err_en_US);
         context_.SetError(symbol);

flang/test/Semantics/bind-c12.f90

+! RUN: %python %S/test_errors.py %s %flang_fc1
+!ERROR: A dummy procedure to an interoperable procedure must also be interoperable
+subroutine subr(e) bind(c)
+  external e
+end