[Flang] [Semantics] More descript warning message for same named external statements and interfaces

[pscoro]

Summary

For issue: https://github.com/llvm/llvm-project/issues/56605. Previous error messages are not descriptive of problem. Issuing warnings instead that do not halt compilation but offer better description of problem.

[klausler]

Looks great to me; thanks! Please consider whether the new overloaded member function SayExternalDeclared will ever have another call site -- if that's unlikely, just call Say() instead and avoid the extra declarations.

[pscoro]

Replaced unneeded SayExternalDeclared methods with Say and fixed typo

flang/lib/Semantics/resolve-names.cpp

return generic->specific() &&
         HandlePreviousCalls(name, *generic->specific(), subpFlag);
   } else if (const auto *proc{symbol.detailsIf<ProcEntityDetails>()}; proc &&
              !proc->isDummy() &&
              !symbol.attrs().HasAny(Attrs{Attr::INTRINSIC, Attr::POINTER})) {
     // There's a symbol created for previous calls to this subprogram or
     // ENTRY's name.  We have to replace that symbol in situ to avoid the
     // obligation to rewrite symbol pointers in the parse tree.
     if (!symbol.test(subpFlag)) {
+      // External statements issue an explicit EXTERNAL attribute.
+      if (symbol.attrs().test(Attr::EXTERNAL) &&
+          !symbol.implicitAttrs().test(Attr::EXTERNAL)) {
+        // Warn if external statement previously declared.
+        Say(name,
+            "EXTERNAL attribute was already specified on '%s'"_warn_en_US);
+      } else {
         Say2(name,
             subpFlag == Symbol::Flag::Function
                 ? "'%s' was previously called as a subroutine"_err_en_US
                 : "'%s' was previously called as a function"_err_en_US,
             symbol, "Previous call of '%s'"_en_US);
       }
+    }
     EntityDetails entity;
     if (proc->type()) {
       entity.set_type(*proc->type());
     }
     symbol.details() = std::move(entity);
     return true;
   } else {
     return symbol.has<UnknownDetails>() || symbol.has<SubprogramNameDetails>();

 bool DeclarationVisitor::Pre(const parser::ContiguousStmt &x) {
   return HandleAttributeStmt(Attr::CONTIGUOUS, x.v);
 }
 bool DeclarationVisitor::Pre(const parser::ExternalStmt &x) {
   HandleAttributeStmt(Attr::EXTERNAL, x.v);
   for (const auto &name : x.v) {
     auto *symbol{FindSymbol(name)};
     if (!ConvertToProcEntity(DEREF(symbol))) {
+      // Check if previous symbol is an interface.
+      if (auto *details{symbol->detailsIf<SubprogramDetails>()}) {
+        if (details->isInterface()) {
+          // Warn if interface previously declared.
+          Say(name,
+              "EXTERNAL attribute was already specified on '%s'"_warn_en_US);
+        }
+      } else {
         SayWithDecl(
             name, *symbol, "EXTERNAL attribute not allowed on '%s'"_err_en_US);
+      }
     } else if (symbol->attrs().test(Attr::INTRINSIC)) { // C840
       Say(symbol->name(),
           "Symbol '%s' cannot have both INTRINSIC and EXTERNAL attributes"_err_en_US,
           symbol->name());
     }
   }
   return false;
 }

flang/test/Semantics/resolve20.f90

module m
 
   !ERROR: EXTERNAL attribute was already specified on 'a'
   !ERROR: EXTERNAL attribute was already specified on 'b'
   !ERROR: EXTERNAL attribute was already specified on 'c'
   !ERROR: EXTERNAL attribute was already specified on 'd'
   external :: a, b, c, d
   !ERROR: EXTERNAL attribute not allowed on 'm'
   external :: m
-  !ERROR: EXTERNAL attribute not allowed on 'foo'
+  !WARNING: EXTERNAL attribute was already specified on 'foo'
   external :: foo
   !ERROR: EXTERNAL attribute not allowed on 'bar'
   external :: bar
 
   !ERROR: PARAMETER attribute not allowed on 'm'
   parameter(m=2)
   !ERROR: PARAMETER attribute not allowed on 'foo'
   parameter(foo=2)