[flang] Downgrade a too-strong error message to a warning (#80095)

When a compilation unit has an interface to an external subroutine or
function, and there is a global object (like a module) with the same
name, we're emitting an error. This is too strong, the program will
still build. This comes up in real applications, too. Downgrade the
error to a warning.

Author: klausler

flang/include/flang/Common/Fortran-features.h

@@ -54,7 +54,7 @@ ENUM_CLASS(UsageWarning, Portability, PointerToUndefinable,
     ShortCharacterActual, ExprPassedToVolatile, ImplicitInterfaceActual,
     PolymorphicTransferArg, PointerComponentTransferArg, TransferSizePresence,
     F202XAllocatableBreakingChange, DimMustBePresent, CommonBlockPadding,
-    LogicalVsCBool, BindCCharLength, ProcDummyArgShapes)
+    LogicalVsCBool, BindCCharLength, ProcDummyArgShapes, ExternalNameConflict)
 
 using LanguageFeatures = EnumSet<LanguageFeature, LanguageFeature_enumSize>;
 using UsageWarnings = EnumSet<UsageWarning, UsageWarning_enumSize>;

flang/lib/Semantics/check-declarations.cpp

@@ -1465,10 +1465,11 @@ void CheckHelper::CheckExternal(const Symbol &symbol) {
       if (interfaceName == definitionName) {
         parser::Message *msg{nullptr};
         if (!IsProcedure(*global)) {
-          if (symbol.flags().test(Symbol::Flag::Function) ||
-              symbol.flags().test(Symbol::Flag::Subroutine)) {
-            msg = messages_.Say(
-                "The global entity '%s' corresponding to the local procedure '%s' is not a callable subprogram"_err_en_US,
+          if ((symbol.flags().test(Symbol::Flag::Function) ||
+                  symbol.flags().test(Symbol::Flag::Subroutine)) &&
+              context_.ShouldWarn(common::UsageWarning::ExternalNameConflict)) {
+            msg = WarnIfNotInModuleFile(
+                "The global entity '%s' corresponding to the local procedure '%s' is not a callable subprogram"_warn_en_US,
                 global->name(), symbol.name());
           }
         } else if (auto chars{Characterize(symbol)}) {

flang/test/Semantics/local-vs-global.f90

@@ -1,4 +1,4 @@
-! RUN: %python %S/test_errors.py %s %flang_fc1
+! RUN: %python %S/test_errors.py %s %flang_fc1 -pedantic
 
 module module_before_1
 end
@@ -46,19 +46,19 @@ function implicit_func_before_2(a)
 
 program test
   external justfine ! OK to name a BLOCK DATA if not called
-  !ERROR: The global entity 'module_before_1' corresponding to the local procedure 'module_before_1' is not a callable subprogram
+  !WARNING: The global entity 'module_before_1' corresponding to the local procedure 'module_before_1' is not a callable subprogram
   external module_before_1
-  !ERROR: The global entity 'block_data_before_1' corresponding to the local procedure 'block_data_before_1' is not a callable subprogram
+  !WARNING: The global entity 'block_data_before_1' corresponding to the local procedure 'block_data_before_1' is not a callable subprogram
   external block_data_before_1
   !ERROR: The global subprogram 'explicit_before_1' may not be referenced via the implicit interface 'explicit_before_1'
   external explicit_before_1
   external implicit_before_1
   !ERROR: The global subprogram 'explicit_func_before_1' may not be referenced via the implicit interface 'explicit_func_before_1'
   external explicit_func_before_1
   external implicit_func_before_1
-  !ERROR: The global entity 'module_after_1' corresponding to the local procedure 'module_after_1' is not a callable subprogram
+  !WARNING: The global entity 'module_after_1' corresponding to the local procedure 'module_after_1' is not a callable subprogram
   external module_after_1
-  !ERROR: The global entity 'block_data_after_1' corresponding to the local procedure 'block_data_after_1' is not a callable subprogram
+  !WARNING: The global entity 'block_data_after_1' corresponding to the local procedure 'block_data_after_1' is not a callable subprogram
   external block_data_after_1
   !ERROR: The global subprogram 'explicit_after_1' may not be referenced via the implicit interface 'explicit_after_1'
   external explicit_after_1