[flang] Respect inaccessibility of type-bound ASSIGNMENT(=)

[klausler]

Summary

When a derived type has a PRIVATE type-bound generic binding for
a defined ASSIGNMENT(=), don't use it in scopes outside of the
module that defines the type. We already get this case right
for other type-bound generics, including defined operators,
and for non-type-bound generic interfaces, but the check was
not applied for this case.

[clementval]

LGTM

[klausler]

Fix bug found during integration testing -- it is the accessibility of the generic that matters, of course, not the specific binding.

flang/lib/Semantics/expression.cpp

std::optional<ProcedureRef> ArgumentAnalyzer::GetDefinedAssignmentProc() {
   parser::CharBlock oprName{oprNameString};
   const auto &scope{context_.context().FindScope(source_)};
   // If multiple resolutions were possible, they will have been already
   // diagnosed.
   {
     auto restorer{context_.GetContextualMessages().DiscardMessages()};
     if (const Symbol *symbol{scope.FindSymbol(oprName)}) {
       ExpressionAnalyzer::AdjustActuals noAdjustment;
-      auto pair{context_.ResolveGeneric(*symbol, actuals_, noAdjustment, true)};
-      if (pair.first) {
-        proc = pair.first;
-      } else {
-        context_.EmitGenericResolutionError(*symbol, pair.second, true);
-      }
+      proc =
+          context_.ResolveGeneric(*symbol, actuals_, noAdjustment, true).first;
     }
     for (std::size_t i{0}; !proc && i < actuals_.size(); ++i) {
       const Symbol *generic{nullptr};
       if (const Symbol *binding{FindBoundOp(oprName, i, generic, true)}) {
-        if (const Symbol *resolution{
-                GetBindingResolution(GetType(i), *binding)}) {
+        if (CheckAccessibleSymbol(scope, DEREF(generic))) {
+          // ignore inaccessible type-bound ASSIGNMENT(=) generic
+        } else if (const Symbol *
+            resolution{GetBindingResolution(GetType(i), *binding)}) {
           proc = resolution;
         } else {
           proc = binding;
           passedObjectIndex = i;
         }
       }
     }
   }