[flang][OpenMP] Verify uses of OmpCancellationConstructTypeClause (llvm#139743)

Some directive names can be used as clauses, for example in "cancel". In
case where a directive name is misplaced, it could be interpreted as a
clause.

Verify that such uses are valid, and emit a diagnostic message if not.

Fixes llvm#138224

Author: kparzysz

flang/lib/Parser/openmp-parsers.cpp

@@ -98,10 +98,12 @@ struct OmpDirectiveNameParser {
   using Token = TokenStringMatch<false, false>;
 
   std::optional<resultType> Parse(ParseState &state) const {
+    auto begin{state.GetLocation()};
     for (const NameWithId &nid : directives()) {
       if (attempt(Token(nid.first.data())).Parse(state)) {
         OmpDirectiveName n;
         n.v = nid.second;
+        n.source = parser::CharBlock(begin, state.GetLocation());
         return n;
       }
     }
@@ -1104,18 +1106,8 @@ TYPE_PARSER( //
     "WHEN" >> construct<OmpClause>(construct<OmpClause::When>(
                   parenthesized(Parser<OmpWhenClause>{}))) ||
     // Cancellable constructs
-    "DO"_id >=
-        construct<OmpClause>(construct<OmpClause::CancellationConstructType>(
-            Parser<OmpCancellationConstructTypeClause>{})) ||
-    "PARALLEL"_id >=
-        construct<OmpClause>(construct<OmpClause::CancellationConstructType>(
-            Parser<OmpCancellationConstructTypeClause>{})) ||
-    "SECTIONS"_id >=
-        construct<OmpClause>(construct<OmpClause::CancellationConstructType>(
-            Parser<OmpCancellationConstructTypeClause>{})) ||
-    "TASKGROUP"_id >=
-        construct<OmpClause>(construct<OmpClause::CancellationConstructType>(
-            Parser<OmpCancellationConstructTypeClause>{})))
+    construct<OmpClause>(construct<OmpClause::CancellationConstructType>(
+        Parser<OmpCancellationConstructTypeClause>{})))
 
 // [Clause, [Clause], ...]
 TYPE_PARSER(sourced(construct<OmpClauseList>(

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

@@ -2422,20 +2422,30 @@ void OmpStructureChecker::Leave(const parser::OpenMPCriticalConstruct &) {
 
 void OmpStructureChecker::Enter(
     const parser::OmpClause::CancellationConstructType &x) {
-  // Do not call CheckAllowed/CheckAllowedClause, because in case of an error
-  // it will print "CANCELLATION_CONSTRUCT_TYPE" as the clause name instead of
-  // the contained construct name.
+  llvm::omp::Directive dir{GetContext().directive};
   auto &dirName{std::get<parser::OmpDirectiveName>(x.v.t)};
-  switch (dirName.v) {
-  case llvm::omp::Directive::OMPD_do:
-  case llvm::omp::Directive::OMPD_parallel:
-  case llvm::omp::Directive::OMPD_sections:
-  case llvm::omp::Directive::OMPD_taskgroup:
-    break;
-  default:
-    context_.Say(dirName.source, "%s is not a cancellable construct"_err_en_US,
-        parser::ToUpperCaseLetters(getDirectiveName(dirName.v).str()));
-    break;
+
+  if (dir != llvm::omp::Directive::OMPD_cancel &&
+      dir != llvm::omp::Directive::OMPD_cancellation_point) {
+    // Do not call CheckAllowed/CheckAllowedClause, because in case of an error
+    // it will print "CANCELLATION_CONSTRUCT_TYPE" as the clause name instead
+    // of the contained construct name.
+    context_.Say(dirName.source, "%s cannot follow %s"_err_en_US,
+        parser::ToUpperCaseLetters(getDirectiveName(dirName.v)),
+        parser::ToUpperCaseLetters(getDirectiveName(dir)));
+  } else {
+    switch (dirName.v) {
+    case llvm::omp::Directive::OMPD_do:
+    case llvm::omp::Directive::OMPD_parallel:
+    case llvm::omp::Directive::OMPD_sections:
+    case llvm::omp::Directive::OMPD_taskgroup:
+      break;
+    default:
+      context_.Say(dirName.source,
+          "%s is not a cancellable construct"_err_en_US,
+          parser::ToUpperCaseLetters(getDirectiveName(dirName.v)));
+      break;
+    }
   }
 }
 

flang/test/Semantics/OpenMP/cancellation-construct-type.f90

@@ -0,0 +1,11 @@
+!RUN: %python %S/../test_errors.py %s %flang_fc1 %openmp_flags
+
+subroutine f(x)
+  integer :: x
+!ERROR: PARALLEL cannot follow SECTIONS
+!$omp sections parallel
+!$omp section
+  x = x + 1
+!$omp end sections
+end
+end