[Flang][OpenMP] Update semantics checks for 'teams' nesting (#126922)

This patch introduces a directive set for combined constructs where
`teams` is the last leaf. This is used in a couple places to simplify
checks, which is NFC, but it also replaces two incorrect uses of
`topTeamsSet`.

Before, these checks would incorrectly skip combined constructs where
`teams` was the last leaf construct when checking for allowed nested
constructs inside of a `teams` region. Similarly, it would also
incorrectly perform these checks whenever a compound `teams` construct
where `teams` was the first leaf construct was found.

Author: skatrak

flang/include/flang/Semantics/openmp-directive-sets.h

@@ -21,6 +21,8 @@ namespace llvm::omp {
 //===----------------------------------------------------------------------===//
 // - top<Directive>Set: The directive appears alone or as the first in a
 //   compound construct.
+// - bottom<Directive>Set: The directive appears alone or as the last in a
+//   compound construct.
 // - all<Directive>Set: All standalone or compound uses of the directive.
 
 static const OmpDirectiveSet topDistributeSet{
@@ -172,6 +174,11 @@ static const OmpDirectiveSet topTeamsSet{
     Directive::OMPD_teams_loop,
 };
 
+static const OmpDirectiveSet bottomTeamsSet{
+    Directive::OMPD_target_teams,
+    Directive::OMPD_teams,
+};
+
 static const OmpDirectiveSet allTeamsSet{
     OmpDirectiveSet{
         Directive::OMPD_target_teams,

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

@@ -487,8 +487,7 @@ void OmpStructureChecker::HasInvalidDistributeNesting(
       violation = true;
     } else {
       // `distribute` region has to be strictly nested inside `teams`
-      if (!OmpDirectiveSet{llvm::omp::OMPD_teams, llvm::omp::OMPD_target_teams}
-              .test(GetContextParent().directive)) {
+      if (!llvm::omp::bottomTeamsSet.test(GetContextParent().directive)) {
         violation = true;
       }
     }
@@ -518,8 +517,7 @@ void OmpStructureChecker::HasInvalidLoopBinding(
 
   if (llvm::omp::Directive::OMPD_loop == beginDir.v &&
       CurrentDirectiveIsNested() &&
-      OmpDirectiveSet{llvm::omp::OMPD_teams, llvm::omp::OMPD_target_teams}.test(
-          GetContextParent().directive)) {
+      llvm::omp::bottomTeamsSet.test(GetContextParent().directive)) {
     teamsBindingChecker(
         "`BIND(TEAMS)` must be specified since the `LOOP` region is "
         "strictly nested inside a `TEAMS` region."_err_en_US);
@@ -726,7 +724,7 @@ void OmpStructureChecker::Enter(const parser::OpenMPLoopConstruct &x) {
   HasInvalidDistributeNesting(x);
   HasInvalidLoopBinding(x);
   if (CurrentDirectiveIsNested() &&
-      llvm::omp::topTeamsSet.test(GetContextParent().directive)) {
+      llvm::omp::bottomTeamsSet.test(GetContextParent().directive)) {
     HasInvalidTeamsNesting(beginDir.v, beginDir.source);
   }
   if ((beginDir.v == llvm::omp::Directive::OMPD_distribute_parallel_do_simd) ||
@@ -1169,7 +1167,7 @@ void OmpStructureChecker::Enter(const parser::OpenMPBlockConstruct &x) {
   }
 
   if (CurrentDirectiveIsNested()) {
-    if (llvm::omp::topTeamsSet.test(GetContextParent().directive)) {
+    if (llvm::omp::bottomTeamsSet.test(GetContextParent().directive)) {
       HasInvalidTeamsNesting(beginDir.v, beginDir.source);
     }
     if (GetContext().directive == llvm::omp::Directive::OMPD_master) {

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

@@ -54,6 +54,7 @@ program main
   n2 = 10
   !$omp target teams map(to:a)
   !PORTABILITY: If TARGET DATA directive is nested inside TARGET region, the behaviour is unspecified
+  !ERROR: Only `DISTRIBUTE`, `PARALLEL`, or `LOOP` regions are allowed to be strictly nested inside `TEAMS` region.
   !$omp target data map(n1,n2)
   do i=1, n1
      do j=1, n2
@@ -65,6 +66,7 @@ program main
 
   !$omp target teams map(to:a) map(from:n1,n2)
   !PORTABILITY: If TARGET TEAMS DISTRIBUTE PARALLEL DO directive is nested inside TARGET region, the behaviour is unspecified
+  !ERROR: Only `DISTRIBUTE`, `PARALLEL`, or `LOOP` regions are allowed to be strictly nested inside `TEAMS` region.
   !$omp target teams distribute parallel do
   do i=1, n1
      do j=1, n2

flang/test/Semantics/OpenMP/nested-teams.f90

@@ -68,6 +68,7 @@ program main
   !$omp end target
 
   !$omp target teams
+  !ERROR: Only `DISTRIBUTE`, `PARALLEL`, or `LOOP` regions are allowed to be strictly nested inside `TEAMS` region.
   !ERROR: TEAMS region can only be strictly nested within the implicit parallel region or TARGET region
   !$omp teams
   a = 3.14