[flang] Clarify edge case of host association and generic interfaces

[klausler]

Summary

Name resolution was mishandling cases of generic interfaces and specific procedures
(sometimes complicatd by use of the same name for each) when the specific procedure
was accessed by means of host association; only the scope of the generic interface
definition was searched for the specific procedure. Also search enclosing scopes
in the usual way.

[klausler]

Address failure found in testing.

flang/lib/Semantics/resolve-names.cpp

if (IsLogicalConstant(context(), symbolName)) {
     Say(symbolName,
         "Logical constant '%s' may not be used as a defined operator"_err_en_US);
     return;
   }
   GenericDetails genericDetails;
   Symbol *existing{nullptr};
   // Check all variants of names, e.g. "operator(.ne.)" for "operator(/=)"
   for (const std::string &n : GetAllNames(context(), symbolName)) {
-    if (auto iter{currScope().find(n)}; iter != currScope().end()) {
-      existing = &*iter->second;
+    existing = currScope().FindSymbol(SourceName{n});
+    if (existing) {
       break;
     }
   }
   if (existing) {
     Symbol &ultimate{existing->GetUltimate()};
     if (auto *existingGeneric{ultimate.detailsIf<GenericDetails>()}) {
       if (const auto *existingUse{existing->detailsIf<UseDetails>()}) {
         // Create a local copy of a use associated generic so that
         // it can be locally extended without corrupting the original.
         genericDetails.CopyFrom(*existingGeneric);
         if (existingGeneric->specific()) {
           genericDetails.set_specific(*existingGeneric->specific());
         }
         AddGenericUse(genericDetails, existing->name(), existingUse->symbol());
-      } else if (existing == &ultimate) {
+      } else if (&existing->owner() == &currScope()) {
+        if (existing == &ultimate) {
           // Extending an extant generic in the same scope
           info.Resolve(existing);
           return;
         } else {
-        // Host association of a generic is handled in ResolveGeneric()
+          // Host association of a generic is handled elsewhere
           CHECK(existing->has<HostAssocDetails>());
         }
+      }
     } else if (ultimate.has<SubprogramDetails>() ||
         ultimate.has<SubprogramNameDetails>()) {
       genericDetails.set_specific(*existing);
     } else if (ultimate.has<DerivedTypeDetails>()) {
       genericDetails.set_derivedType(*existing);
-    } else {
+    } else if (&existing->owner() == &currScope()) {
       SayAlreadyDeclared(symbolName, *existing);
       return;
     }
+    if (&existing->owner() == &currScope()) {
       EraseSymbol(*existing);
     }
+  }
   info.Resolve(&MakeSymbol(symbolName, Attrs{}, std::move(genericDetails)));
 }
 
 void ResolveNamesVisitor::FinishSpecificationPart(
     const std::list<parser::DeclarationConstruct> &decls) {
   badStmtFuncFound_ = false;
   funcResultStack().CompleteFunctionResultType();
   CheckImports();

flang/test/Semantics/generic01.f90

block
       !CHECK: 1.41421353816986083984375_4
       print *, abs((1,1))
       !CHECK: abs_noargs(
       print *, abs()
     end block
   end subroutine
   subroutine test3
     interface abs
-      module procedure abs_int_redef2 ! override module's use of m1
+      module procedure abs_complex_redef ! extend module's use of m1
     end interface
-    !CHECK: abs_int_redef2(
+    !CHECK: abs_int_redef(
     print *, abs(1)
     !CHECK: 1._4
     print *, abs(1.)
-    !CHECK: 1.41421353816986083984375_4
+    !CHECK: abs_complex_redef(
     print *, abs((1,1))
     !CHECK: abs_noargs(
     print *, abs()
     block
-      use m1, only: abs ! override the override
-      !CHECK: abs_int_redef(
-      print *, abs(1)
+      intrinsic abs ! override the extension
+      !CHECK: 1.41421353816986083984375_4
+      print *, abs((1,1))
     end block
   end subroutine
-  integer function abs_int_redef2(j)
-    integer, intent(in) :: j
-    abs_int_redef2 = j
+  real function abs_complex_redef(z)
+    complex, intent(in) :: z
+    abs_complex_redef = z
+  end function
+  subroutine test4
+    !CHECK: abs(
+    print *, abs(1)
+   contains
+    integer function abs(n) ! override module's use of m1
+      integer, intent(in) :: n
+      abs = n
     end function
+  end subroutine
+end module
+
+module m4
+ contains
+  integer function abs(n)
+    integer, intent(in) :: n
+    abs = n
+  end function
+  subroutine test5
+    interface abs
+      module procedure abs ! same name, host-associated
+    end interface
+    !CHECK: abs(
+    print *, abs(1)
+  end subroutine
 end module