[Flang][OpenMP] Give better errors during parsing failures (#90480)

Similar to OpenACC handling.

Fixes #90452

Author: kiranchandramohan

flang/lib/Parser/openmp-parsers.cpp

@@ -634,18 +634,20 @@ TYPE_PARSER(
 
 // Declarative constructs
 TYPE_PARSER(startOmpLine >>
-    sourced(construct<OpenMPDeclarativeConstruct>(
-                Parser<OpenMPDeclareReductionConstruct>{}) ||
-        construct<OpenMPDeclarativeConstruct>(
-            Parser<OpenMPDeclareSimdConstruct>{}) ||
-        construct<OpenMPDeclarativeConstruct>(
-            Parser<OpenMPDeclareTargetConstruct>{}) ||
-        construct<OpenMPDeclarativeConstruct>(
-            Parser<OpenMPDeclarativeAllocate>{}) ||
-        construct<OpenMPDeclarativeConstruct>(
-            Parser<OpenMPRequiresConstruct>{}) ||
-        construct<OpenMPDeclarativeConstruct>(Parser<OpenMPThreadprivate>{})) /
-        endOmpLine)
+    withMessage("expected OpenMP construct"_err_en_US,
+        sourced(construct<OpenMPDeclarativeConstruct>(
+                    Parser<OpenMPDeclareReductionConstruct>{}) ||
+            construct<OpenMPDeclarativeConstruct>(
+                Parser<OpenMPDeclareSimdConstruct>{}) ||
+            construct<OpenMPDeclarativeConstruct>(
+                Parser<OpenMPDeclareTargetConstruct>{}) ||
+            construct<OpenMPDeclarativeConstruct>(
+                Parser<OpenMPDeclarativeAllocate>{}) ||
+            construct<OpenMPDeclarativeConstruct>(
+                Parser<OpenMPRequiresConstruct>{}) ||
+            construct<OpenMPDeclarativeConstruct>(
+                Parser<OpenMPThreadprivate>{})) /
+            endOmpLine))
 
 // Block Construct
 TYPE_PARSER(construct<OpenMPBlockConstruct>(
@@ -681,17 +683,18 @@ TYPE_PARSER(construct<OpenMPSectionsConstruct>(
 
 TYPE_CONTEXT_PARSER("OpenMP construct"_en_US,
     startOmpLine >>
-        first(construct<OpenMPConstruct>(Parser<OpenMPSectionsConstruct>{}),
-            construct<OpenMPConstruct>(Parser<OpenMPLoopConstruct>{}),
-            construct<OpenMPConstruct>(Parser<OpenMPBlockConstruct>{}),
-            // OpenMPBlockConstruct is attempted before
-            // OpenMPStandaloneConstruct to resolve !$OMP ORDERED
-            construct<OpenMPConstruct>(Parser<OpenMPStandaloneConstruct>{}),
-            construct<OpenMPConstruct>(Parser<OpenMPAtomicConstruct>{}),
-            construct<OpenMPConstruct>(Parser<OpenMPExecutableAllocate>{}),
-            construct<OpenMPConstruct>(Parser<OpenMPAllocatorsConstruct>{}),
-            construct<OpenMPConstruct>(Parser<OpenMPDeclarativeAllocate>{}),
-            construct<OpenMPConstruct>(Parser<OpenMPCriticalConstruct>{})))
+        withMessage("expected OpenMP construct"_err_en_US,
+            first(construct<OpenMPConstruct>(Parser<OpenMPSectionsConstruct>{}),
+                construct<OpenMPConstruct>(Parser<OpenMPLoopConstruct>{}),
+                construct<OpenMPConstruct>(Parser<OpenMPBlockConstruct>{}),
+                // OpenMPBlockConstruct is attempted before
+                // OpenMPStandaloneConstruct to resolve !$OMP ORDERED
+                construct<OpenMPConstruct>(Parser<OpenMPStandaloneConstruct>{}),
+                construct<OpenMPConstruct>(Parser<OpenMPAtomicConstruct>{}),
+                construct<OpenMPConstruct>(Parser<OpenMPExecutableAllocate>{}),
+                construct<OpenMPConstruct>(Parser<OpenMPAllocatorsConstruct>{}),
+                construct<OpenMPConstruct>(Parser<OpenMPDeclarativeAllocate>{}),
+                construct<OpenMPConstruct>(Parser<OpenMPCriticalConstruct>{}))))
 
 // END OMP Block directives
 TYPE_PARSER(

flang/test/Parser/OpenMP/fail-construct1.f90

@@ -0,0 +1,5 @@
+! RUN: not %flang_fc1 -fsyntax-only -fopenmp %s 2>&1 | FileCheck %s
+
+! CHECK: error: expected OpenMP construct
+!$omp  parallel
+end

flang/test/Parser/OpenMP/fail-construct2.f90

@@ -0,0 +1,5 @@
+! RUN: not %flang_fc1 -fsyntax-only -fopenmp %s 2>&1 | FileCheck %s
+
+! CHECK: error: expected OpenMP construct
+!$omp dummy
+end