Fix missing group completion proposal (#1689)

Author: ydaveluy

packages/langium/src/lsp/completion/follow-element-computation.ts

@@ -52,7 +52,9 @@ function findNextFeaturesInternal(options: { next: NextFeature, cardinalities: M
     const feature = next.feature;
     if (visited.has(feature)) {
         return [];
-    } else {
+    } else if (!ast.isGroup(feature)) {
+        // Do not add the feature to the list if it is a group
+        // `findFirstFeaturesInternal` will take care of this
         visited.add(feature);
     }
     let parent: ast.Group | undefined;
@@ -132,8 +134,6 @@ function findFirstFeaturesInternal(options: { next: NextFeature, cardinalities:
         } else {
             visited.add(feature);
         }
-    }
-    if (ast.isGroup(feature)) {
         return findNextFeaturesInGroup(next as NextFeature<ast.Group>, 0, cardinalities, visited, plus)
             .map(e => modifyCardinality(e, feature.cardinality, cardinalities));
     } else if (ast.isAlternatives(feature) || ast.isUnorderedGroup(feature)) {

packages/langium/test/lsp/completion-provider.test.ts

@@ -131,7 +131,35 @@ describe('Completion within alternatives', () => {
             expectedItems: ['c', 'd']
         });
     });
+    test('Should show correct keywords in completion of group', async () => {
 
+        const grammar = `
+        grammar g
+        entry Main: ('a' a+=ID | 'b' b+=ID | 'c' c+=ID)*;
+        hidden terminal WS: /\\s+/;
+        terminal ID: /\\w+/;
+        `;
+
+        const services = await createServicesForGrammar({ grammar });
+        const completion = expectCompletion(services);
+        const text = '<|>a id1 <|>b id2 <|>c id3';
+
+        await completion({
+            text,
+            index: 0,
+            expectedItems: ['a', 'b', 'c']
+        });
+        await completion({
+            text,
+            index: 1,
+            expectedItems: ['a', 'b', 'c']
+        });
+        await completion({
+            text,
+            index: 2,
+            expectedItems: ['a', 'b', 'c']
+        });
+    });
     test('Should show correct cross reference and keyword in completion', async () => {
 
         const grammar = `