[flang][OpenMP] Show error for task depend with no valid modifiers (#142595)

If a "TASK DEPEND" clause is not given a valid task dependece type
modifier, the semantic checks for the clause will result in an ICE
because they assume that such modifiers will be present. Check whether
the modifiers are present and show an appropriate error instead of
crashing the compiler if they are not.

Fixes #133678.

Signed-off-by: Kajetan Puchalski <kajetan.puchalski@arm.com>

Author: mrkajetanp

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

@@ -4420,6 +4420,13 @@ void OmpStructureChecker::Enter(const parser::OmpClause::Depend &x) {
     CheckDoacross(*doaDep);
     CheckDependenceType(doaDep->GetDepType());
   } else {
+    using Modifier = parser::OmpDependClause::TaskDep::Modifier;
+    auto &modifiers{std::get<std::optional<std::list<Modifier>>>(taskDep->t)};
+    if (!modifiers) {
+      context_.Say(GetContext().clauseSource,
+          "A DEPEND clause on a TASK construct must have a valid task dependence type"_err_en_US);
+      return;
+    }
     CheckTaskDependenceType(taskDep->GetTaskDepType());
   }
 

flang/test/Semantics/OpenMP/task-depend.f90

@@ -0,0 +1,8 @@
+! RUN: %python %S/../test_errors.py %s %flang -fopenmp
+
+program test
+! ERROR: A DEPEND clause on a TASK construct must have a valid task dependence type
+!$omp task depend(ii)
+!$omp end task
+end
+