[flang] Further refine errors vs warnings for ambiguous generics

Ensure that the compiler emits a hard error for a generic interface
with ambiguous specific procedures when it is declared as such, and
the ambiguity doesn't involve optional or unlimited polymorphic dummy
data arguments.  But: emit an optional portability warning when the
ambiguity in the generic interface is due to USE association's merging
of multiple generics, as USE association may involve modules not under
control of the programmer; we'll emit a hard error message if any
the actual arguments in a particular reference to the generic procedure
doesn't resolve to exactly one specific procedure.  And don't emit
warnings when potential ambiguity due to USE association is taking
place in a module file; the warnings, if any, will have been produced
when the module file was compiled.

Author: klausler

flang/lib/Semantics/check-declarations.cpp

@@ -192,7 +192,7 @@ class DistinguishabilityHelper {
 
 private:
   void SayNotDistinguishable(const Scope &, const SourceName &, GenericKind,
-      const Symbol &, const Symbol &, bool isError);
+      const Symbol &, const Symbol &, bool isHardConflict);
   void AttachDeclaration(parser::Message &, const Scope &, const Symbol &);
 
   SemanticsContext &context_;
@@ -3512,6 +3512,11 @@ void DistinguishabilityHelper::Add(const Symbol &generic, GenericKind kind,
 }
 
 void DistinguishabilityHelper::Check(const Scope &scope) {
+  if (FindModuleFileContaining(scope)) {
+    // Distinguishability was checked when the module was created;
+    // don't let optional warnings then become errors now.
+    return;
+  }
   for (const auto &[name, info] : nameToSpecifics_) {
     for (auto iter1{info.begin()}; iter1 != info.end(); ++iter1) {
       const auto &[ultimate, procInfo]{*iter1};
@@ -3533,15 +3538,19 @@ void DistinguishabilityHelper::Check(const Scope &scope) {
 
 void DistinguishabilityHelper::SayNotDistinguishable(const Scope &scope,
     const SourceName &name, GenericKind kind, const Symbol &proc1,
-    const Symbol &proc2, bool isError) {
-  if (!isError &&
-      !context_.ShouldWarn(
-          common::LanguageFeature::IndistinguishableSpecifics)) {
-    // The rules for distinguishing specific procedures (F'2023 15.4.3.4.5)
-    // are inadequate for some real-world cases like pFUnit.
-    // When there are optional dummy arguments or unlimited polymorphic
-    // dummy data object arguments, the best that we can do is emit an optional
-    // portability warning.
+    const Symbol &proc2, bool isHardConflict) {
+  bool isUseAssociated{!scope.sourceRange().Contains(name)};
+  // The rules for distinguishing specific procedures (F'2023 15.4.3.4.5)
+  // are inadequate for some real-world cases like pFUnit.
+  // When there are optional dummy arguments or unlimited polymorphic
+  // dummy data object arguments, the best that we can do is emit an optional
+  // portability warning.  Also, generics created by USE association
+  // merging shouldn't receive hard errors for ambiguity.
+  bool isWarning{!isHardConflict || isUseAssociated};
+  if (isWarning &&
+      (!context_.ShouldWarn(
+           common::LanguageFeature::IndistinguishableSpecifics) ||
+          FindModuleFileContaining(scope))) {
     return;
   }
   std::string name1{proc1.name().ToString()};
@@ -3556,17 +3565,19 @@ void DistinguishabilityHelper::SayNotDistinguishable(const Scope &scope,
     }
   }
   parser::Message *msg;
-  if (scope.sourceRange().Contains(name)) {
+  if (!isUseAssociated) {
+    CHECK(isWarning == !isHardConflict);
     msg = &context_.Say(name,
-        isError
+        isHardConflict
             ? "Generic '%s' may not have specific procedures '%s' and '%s' as their interfaces are not distinguishable"_err_en_US
             : "Generic '%s' should not have specific procedures '%s' and '%s' as their interfaces are not distinguishable by the rules in the standard"_port_en_US,
         MakeOpName(name), name1, name2);
   } else {
+    CHECK(isWarning);
     msg = &context_.Say(*GetTopLevelUnitContaining(proc1).GetName(),
-        isError
-            ? "USE-associated generic '%s' may not have specific procedures '%s' and '%s' as their interfaces are not distinguishable"_err_en_US
-            : "USE-associated generic '%s' should not have specific procedures '%s' and '%s' as their interfaces are not distinguishable by the incomplete rules in the standard"_port_en_US,
+        isHardConflict
+            ? "USE-associated generic '%s' should not have specific procedures '%s' and '%s' as their interfaces are not distinguishable"_warn_en_US
+            : "USE-associated generic '%s' should not have specific procedures '%s' and '%s' as their interfaces are not distinguishable by the rules in the standard"_port_en_US,
         MakeOpName(name), name1, name2);
   }
   AttachDeclaration(*msg, scope, proc1);

flang/test/Semantics/resolve17.f90

@@ -180,7 +180,7 @@ subroutine g()
   end
 end module
 subroutine s9
-  !ERROR: USE-associated generic 'g' may not have specific procedures 'g' and 'g' as their interfaces are not distinguishable
+  !PORTABILITY: USE-associated generic 'g' should not have specific procedures 'g' and 'g' as their interfaces are not distinguishable
   use m9a
   use m9b
 end