[flang] Fix bogus branch target error on END SELECT

[klausler]

Summary

The scope model used for branch target checking treats a label
on an END SELECT statement as if it were in the previous CASE block.
This makes it illegal to GO TO that label from within any earlier
CASE block in that statement. Fix by treating the CASE blocks as
nested scopes within the scope of the SELECT construct.

Also, add a "warning:" tag to related warning messages.

flang/lib/Semantics/resolve-labels.cpp

     using LabeledConstructEndStmts = std::tuple<parser::EndAssociateStmt,
         parser::EndBlockStmt, parser::EndChangeTeamStmt,
         parser::EndCriticalStmt, parser::EndDoStmt, parser::EndForallStmt,
-        parser::EndIfStmt, parser::EndSelectStmt, parser::EndWhereStmt>;
+        parser::EndIfStmt, parser::EndWhereStmt>;
     using LabeledProgramUnitEndStmts =
         std::tuple<parser::EndFunctionStmt, parser::EndMpSubprogramStmt,
             parser::EndProgramStmt, parser::EndSubroutineStmt>;
     auto targetFlags{ConstructBranchTargetFlags(statement)};
     if constexpr (common::HasMember<A, LabeledConstructStmts>) {
       AddTargetLabelDefinition(label.value(), targetFlags, ParentScope());
+    } else if constexpr (std::is_same_v<A, parser::EndSelectStmt>) {
+      // the label on an END SELECT is not in the last case
+      AddTargetLabelDefinition(label.value(), targetFlags, ParentScope(), true);
     } else if constexpr (common::HasMember<A, LabeledConstructEndStmts>) {
       constexpr bool isExecutableConstructEndStmt{true};
       AddTargetLabelDefinition(label.value(), targetFlags, currentScope_,

   bool Pre(const parser::CaseConstruct &caseConstruct) {
     return PushConstructName(caseConstruct);
   }
+  void Post(const parser::SelectCaseStmt &) { PushScope(); }
   bool Pre(const parser::CaseConstruct::Case &) { return SwitchToNewScope(); }
   bool Pre(const parser::SelectRankConstruct &selectRankConstruct) {
     return PushConstructName(selectRankConstruct);
   }
+  void Post(const parser::SelectRankStmt &) { PushScope(); }
   bool Pre(const parser::SelectRankConstruct::RankCase &) {
     return SwitchToNewScope();
   }
   bool Pre(const parser::SelectTypeConstruct &selectTypeConstruct) {
     return PushConstructName(selectTypeConstruct);
   }
+  void Post(const parser::SelectTypeStmt &) { PushScope(); }
   bool Pre(const parser::SelectTypeConstruct::TypeCase &) {
     return SwitchToNewScope();
   }
+  void Post(const parser::EndSelectStmt &) { PopScope(); }
   bool Pre(const parser::WhereConstruct &whereConstruct) {
     return PushConstructName(whereConstruct);
   }

       context.Say(position,
           isFatal
               ? "Label '%u' is in a construct that prevents its use as a branch target here"_err_en_US
-              : "Label '%u' is in a construct that prevents its use as a branch target here"_en_US,
+              : "Label '%u' is in a construct that should not be used as a branch target here"_warn_en_US,
           SayLabel(label));
     }
   }

flang/test/Semantics/label05.f90

 ! RUN: not %flang_fc1 -fdebug-unparse-with-symbols %s 2>&1 | FileCheck %s
 ! CHECK: Label '50' was not found
-! CHECK-NOT: error: Label '55' is in a construct that prevents its use as a branch target here
-! CHECK: Label '55' is in a construct that prevents its use as a branch target here
+! CHECK: warning: Label '55' is in a construct that should not be used as a branch target here
 ! CHECK: Label '70' is not a branch target
 ! CHECK: Control flow use of '70'
 ! CHECK: error: Label '80' is in a construct that prevents its use as a branch target here

flang/test/Semantics/label06.f90

 ! RUN: not %flang_fc1 -fdebug-unparse-with-symbols %s 2>&1 | FileCheck %s
-! CHECK: Label '10' is in a construct that prevents its use as a branch target here
+! CHECK: warning: Label '10' is in a construct that should not be used as a branch target here
 ! CHECK: Label '20' was not found
 ! CHECK: Label '30' is not a branch target
 ! CHECK: Control flow use of '30'
-! CHECK: Label '40' is in a construct that prevents its use as a branch target here
-! CHECK: Label '50' is in a construct that prevents its use as a branch target here
+! CHECK: warning: Label '40' is in a construct that should not be used as a branch target here
+! CHECK: warning: Label '50' is in a construct that should not be used as a branch target here
 
 subroutine sub00(n)
   GOTO (10,20,30) n

flang/test/Semantics/label07.f90

 ! RUN: not %flang_fc1 -fdebug-unparse-with-symbols %s 2>&1 | FileCheck %s
 ! CHECK: Label '30' is not a branch target
 ! CHECK: Control flow use of '30'
-! CHECK: Label '10' is in a construct that prevents its use as a branch target here
+! CHECK: warning: Label '10' is in a construct that should not be used as a branch target here
 ! CHECK: Label '20' was not found
 ! CHECK: Label '60' was not found