[flang] Allow GOTO to containing END IF after ELSE

[klausler]

Summary

Label resolution gets into an infinite loop trying to emit an inappropriate
error or warning for a GOTO whose target is on an enclosing END IF
statement with an intervening ELSE or ELSE IF. The scope tracking mechanism
viewed the END IF as being part of the ELSE block's scope.

Fix with the same means that was used to fix a similar bogus error
on GOTOs to END SELECT in SELECT CASE blocks: nest the THEN/ELSE IF/ELSE
blocks one level deeper than before, so that the END IF is in the IF
block but not in any of its parts.

Fixes https://github.com/llvm/llvm-project/issues/64654 for
llvm-test-suite/Fortran/gfortran/regression/goto_5.f90.

[clementval]

LGTM

[klausler]

Add omitted test case.

flang/lib/Semantics/resolve-labels.cpp

using LabeledConstructEndStmts = std::tuple<parser::EndAssociateStmt,
         parser::EndCriticalStmt, parser::EndDoStmt, parser::EndForallStmt,
         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
+    } else if constexpr (std::is_same_v<A, parser::EndIfStmt> ||
+        std::is_same_v<A, parser::EndSelectStmt>) {
+      // the label on an END IF/SELECT is not in the last part/case
       AddTargetLabelDefinition(label.value(), targetFlags, ParentScope(), true);
     } else if constexpr (common::HasMember<A, LabeledConstructEndStmts>) {
       constexpr bool isExecutableConstructEndStmt{true};
       AddTargetLabelDefinition(label.value(), targetFlags, currentScope_,
           isExecutableConstructEndStmt);
     } else if constexpr (!common::HasMember<A, LabeledProgramUnitEndStmts>) {
       // Program unit END statements have already been processed.
       AddTargetLabelDefinition(label.value(), targetFlags, currentScope_);
bool Pre(const parser::CriticalConstruct &criticalConstruct) {
     return PushConstructName(criticalConstruct);
   }
   bool Pre(const parser::DoConstruct &doConstruct) {
     return PushConstructName(doConstruct);
   }
   bool Pre(const parser::IfConstruct &ifConstruct) {
     return PushConstructName(ifConstruct);
   }
+  void Post(const parser::IfThenStmt &) { PushScope(); }
   bool Pre(const parser::IfConstruct::ElseIfBlock &) {
     return SwitchToNewScope();
   }
   bool Pre(const parser::IfConstruct::ElseBlock &) {
     return SwitchToNewScope();
   }
+  bool Pre(const parser::EndIfStmt &) {
+    PopScope();
+    return true;
+  }
   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);
   }
if (!HasScope(target.proxyForScope)) {
       // C1038 and C1034 forbid statements in FORALL and WHERE constructs
       // (resp.) from being branch targets.
       if (target.labeledStmtClassificationSet.test(
               TargetStatementEnum::Format)) {
         continue;
       }
       bool isFatal{false};
       ProxyForScope fromScope{scope};
-      for (ProxyForScope toScope{target.proxyForScope}; fromScope != toScope;
+      for (ProxyForScope toScope{target.proxyForScope}; HasScope(toScope);
            toScope = scopes[toScope].parent) {
+        while (scopes[fromScope].depth > scopes[toScope].depth) {
+          fromScope = scopes[fromScope].parent;
+        }
+        if (toScope == fromScope) {
+          break;
+        }
         if (scopes[toScope].isExteriorGotoFatal) {
           isFatal = true;
           break;
         }
-        if (scopes[toScope].depth == scopes[fromScope].depth) {
-          fromScope = scopes[fromScope].parent;
-        }
       }
       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 should not be used as a branch target here"_warn_en_US,
           SayLabel(label));
     }
   }

flang/test/Semantics/label18.f90

+! RUN: %python %S/test_errors.py %s %flang_fc1 -Werror
+program main
+  if (.true.) then
+    do j = 1, 2
+      goto 1 ! ok; used to cause looping in label resolution
+    end do
+  else
+    goto 1 ! ok
+1 end if
+  if (.true.) then
+    do j = 1, 2
+      !WARNING: Label '1' is in a construct that should not be used as a branch target here
+      goto 1
+    end do
+  end if
+  !WARNING: Label '1' is in a construct that should not be used as a branch target here
+  goto 1
+end