[Flang][OpenMP] Fix a corner case where target region is empty

[raghavendhra]

flang/lib/Semantics/check-omp-structure.cpp

2642–2656

Nit : Tabs here, Can you please change them to spaces? Otherwise LGTM.

[kiranchandramohan]

flang/lib/Semantics/check-omp-structure.cpp

2642–2656

@raghavendhra Thanks for having a look. I did not see tabs when I fetched this patch locally. Also, clang-format has passed in the CI, clang-format converts tabs to spaces and would have issued an error if it saw a diff after running. So I guess this is OK.
If I have missed something, feel free to submit a fix.

flang/lib/Semantics/check-omp-structure.cpp

if (auto *enclosingContext{GetEnclosingContextWithDir(enclosingDir)}) {
       }
     }
   }
 }
 
 bool OmpStructureChecker::CheckTargetBlockOnlyTeams(
     const parser::Block &block) {
   bool nestedTeams{false};
-  auto it{block.begin()};
 
-  if (const auto *ompConstruct{parser::Unwrap<parser::OpenMPConstruct>(*it)}) {
+  if (!block.empty()) {
+    auto it{block.begin()};
+    if (const auto *ompConstruct{
+            parser::Unwrap<parser::OpenMPConstruct>(*it)}) {
       if (const auto *ompBlockConstruct{
               std::get_if<parser::OpenMPBlockConstruct>(&ompConstruct->u)}) {
         const auto &beginBlockDir{
             std::get<parser::OmpBeginBlockDirective>(ompBlockConstruct->t)};
         const auto &beginDir{
             std::get<parser::OmpBlockDirective>(beginBlockDir.t)};
         if (beginDir.v == llvm::omp::Directive::OMPD_teams) {
           nestedTeams = true;
         }
       }
     }
 
     if (nestedTeams && ++it == block.end()) {
       return true;
     }
+  }
+
   return false;
 }
 
 void OmpStructureChecker::CheckWorkshareBlockStmts(
     const parser::Block &block, parser::CharBlock source) {
   OmpWorkshareBlockChecker ompWorkshareBlockChecker{context_, source};
 
   for (auto it{block.begin()}; it != block.end(); ++it) {

flang/test/Semantics/OpenMP/omp-target.f90

+! RUN: %python %S/../test_errors.py %s %flang_fc1 -fopenmp -Werror
+
+! Corner cases in OpenMP target directives
+
+subroutine empty_target()
+  integer :: i
+
+  !$omp target map(i)
+  !$omp end target
+
+end subroutine