[Flang][OpenMP] Issue an error for loop directive without a loop (#118039)

Fixes #107500

Author: kiranchandramohan

flang/lib/Semantics/resolve-directives.cpp

@@ -1895,22 +1895,29 @@ void OmpAttributeVisitor::PrivatizeAssociatedLoopIndexAndCheckLoopLevel(
   }
 
   const auto &outer{std::get<std::optional<parser::DoConstruct>>(x.t)};
-  for (const parser::DoConstruct *loop{&*outer}; loop && level > 0; --level) {
-    // go through all the nested do-loops and resolve index variables
-    const parser::Name *iv{GetLoopIndex(*loop)};
-    if (iv) {
-      if (auto *symbol{ResolveOmp(*iv, ivDSA, currScope())}) {
-        symbol->set(Symbol::Flag::OmpPreDetermined);
-        iv->symbol = symbol; // adjust the symbol within region
-        AddToContextObjectWithDSA(*symbol, ivDSA);
-      }
+  if (outer.has_value()) {
+    for (const parser::DoConstruct *loop{&*outer}; loop && level > 0; --level) {
+      // go through all the nested do-loops and resolve index variables
+      const parser::Name *iv{GetLoopIndex(*loop)};
+      if (iv) {
+        if (auto *symbol{ResolveOmp(*iv, ivDSA, currScope())}) {
+          symbol->set(Symbol::Flag::OmpPreDetermined);
+          iv->symbol = symbol; // adjust the symbol within region
+          AddToContextObjectWithDSA(*symbol, ivDSA);
+        }
 
-      const auto &block{std::get<parser::Block>(loop->t)};
-      const auto it{block.begin()};
-      loop = it != block.end() ? GetDoConstructIf(*it) : nullptr;
+        const auto &block{std::get<parser::Block>(loop->t)};
+        const auto it{block.begin()};
+        loop = it != block.end() ? GetDoConstructIf(*it) : nullptr;
+      }
     }
+    CheckAssocLoopLevel(level, GetAssociatedClause());
+  } else {
+    context_.Say(GetContext().directiveSource,
+        "A DO loop must follow the %s directive"_err_en_US,
+        parser::ToUpperCaseLetters(
+            llvm::omp::getOpenMPDirectiveName(GetContext().directive).str()));
   }
-  CheckAssocLoopLevel(level, GetAssociatedClause());
 }
 void OmpAttributeVisitor::CheckAssocLoopLevel(
     std::int64_t level, const parser::OmpClause *clause) {

flang/test/Semantics/OpenMP/do21.f90

@@ -0,0 +1,27 @@
+! RUN: %python %S/../test_errors.py %s %flang -fopenmp
+! Check for existence of loop following a DO directive
+
+subroutine do1
+  !ERROR: A DO loop must follow the DO directive
+  !$omp do
+end subroutine
+
+subroutine do2
+  !ERROR: A DO loop must follow the PARALLEL DO directive
+  !$omp parallel do
+end subroutine
+
+subroutine do3
+  !ERROR: A DO loop must follow the SIMD directive
+  !$omp simd
+end subroutine
+
+subroutine do4
+  !ERROR: A DO loop must follow the DO SIMD directive
+  !$omp do simd
+end subroutine
+
+subroutine do5
+  !ERROR: A DO loop must follow the LOOP directive
+  !$omp loop
+end subroutine