[flang][OpenMP] move omp end sections validation to semantics

See #90452. The old parse tree errors exploded to thousands of unhelpful
lines when there were multiple missing end directives.

Instead, allow a missing end directive in the parse tree then validate
that it is present during semantics (where the error messages are a lot
easier to control).

Author: tblah

flang/include/flang/Parser/parse-tree.h

@@ -4893,8 +4893,11 @@ struct OpenMPSectionsConstruct {
   CharBlock source;
   // Each of the OpenMPConstructs in the list below contains an
   // OpenMPSectionConstruct. This is guaranteed by the parser.
+  // The end sections directive is optional here because it is difficult to
+  // generate helpful error messages for a missing end directive wihtin the
+  // parser. Semantics will generate an error if this is absent.
   std::tuple<OmpBeginSectionsDirective, std::list<OpenMPConstruct>,
-      OmpEndSectionsDirective>
+      std::optional<OmpEndSectionsDirective>>
       t;
 };
 

flang/lib/Lower/OpenMP/OpenMP.cpp

@@ -3958,9 +3958,12 @@ static void genOMP(lower::AbstractConverter &converter, lower::SymMap &symTable,
   List<Clause> clauses = makeClauses(
       std::get<parser::OmpClauseList>(beginSectionsDirective.t), semaCtx);
   const auto &endSectionsDirective =
-      std::get<parser::OmpEndSectionsDirective>(sectionsConstruct.t);
+      std::get<std::optional<parser::OmpEndSectionsDirective>>(
+          sectionsConstruct.t);
+  assert(endSectionsDirective &&
+         "Missing end section directive should have been handled in semantics");
   clauses.append(makeClauses(
-      std::get<parser::OmpClauseList>(endSectionsDirective.t), semaCtx));
+      std::get<parser::OmpClauseList>(endSectionsDirective->t), semaCtx));
   mlir::Location currentLocation = converter.getCurrentLocation();
 
   llvm::omp::Directive directive =

flang/lib/Parser/openmp-parsers.cpp

@@ -1917,7 +1917,7 @@ TYPE_PARSER(sourced(construct<OpenMPSectionsConstruct>(
             construct<OpenMPSectionConstruct>(maybe(sectionDir), block))),
         many(construct<OpenMPConstruct>(
             sourced(construct<OpenMPSectionConstruct>(sectionDir, block))))),
-    Parser<OmpEndSectionsDirective>{} / endOmpLine)))
+    maybe(Parser<OmpEndSectionsDirective>{} / endOmpLine))))
 
 static bool IsExecutionPart(const OmpDirectiveName &name) {
   return name.IsExecutionPart();

flang/lib/Parser/unparse.cpp

@@ -2788,7 +2788,7 @@ class UnparseVisitor {
     Walk(std::get<std::list<OpenMPConstruct>>(x.t), "");
     BeginOpenMP();
     Word("!$OMP END ");
-    Walk(std::get<OmpEndSectionsDirective>(x.t));
+    Walk(std::get<std::optional<OmpEndSectionsDirective>>(x.t));
     Put("\n");
     EndOpenMP();
   }

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

@@ -1063,14 +1063,23 @@ void OmpStructureChecker::Leave(const parser::OmpBeginDirective &) {
 void OmpStructureChecker::Enter(const parser::OpenMPSectionsConstruct &x) {
   const auto &beginSectionsDir{
       std::get<parser::OmpBeginSectionsDirective>(x.t)};
-  const auto &endSectionsDir{std::get<parser::OmpEndSectionsDirective>(x.t)};
+  const auto &endSectionsDir{
+      std::get<std::optional<parser::OmpEndSectionsDirective>>(x.t)};
   const auto &beginDir{
       std::get<parser::OmpSectionsDirective>(beginSectionsDir.t)};
-  const auto &endDir{std::get<parser::OmpSectionsDirective>(endSectionsDir.t)};
+  PushContextAndClauseSets(beginDir.source, beginDir.v);
+
+  if (!endSectionsDir) {
+    context_.Say(beginSectionsDir.source,
+        "Expected OpenMP END SECTIONS directive"_err_en_US);
+    // Following code assumes the option is present.
+    return;
+  }
+
+  const auto &endDir{std::get<parser::OmpSectionsDirective>(endSectionsDir->t)};
   CheckMatching<parser::OmpSectionsDirective>(beginDir, endDir);
 
-  PushContextAndClauseSets(beginDir.source, beginDir.v);
-  AddEndDirectiveClauses(std::get<parser::OmpClauseList>(endSectionsDir.t));
+  AddEndDirectiveClauses(std::get<parser::OmpClauseList>(endSectionsDir->t));
 
   const auto &sectionBlocks{std::get<std::list<parser::OpenMPConstruct>>(x.t)};
   for (const parser::OpenMPConstruct &construct : sectionBlocks) {

flang/test/Semantics/OpenMP/missing-end-directive.f90

@@ -6,8 +6,12 @@
 !$omp parallel
 ! ERROR: Expected OpenMP end directive
 !$omp task
+! ERROR: Expected OpenMP END SECTIONS directive
+!$omp sections
 ! ERROR: Expected OpenMP end directive
 !$omp parallel
 ! ERROR: Expected OpenMP end directive
 !$omp task
+! ERROR: Expected OpenMP END SECTIONS directive
+!$omp sections
 end