Fix unassigned data type rule usage (#1367)

Author: msujew

packages/langium/src/parser/langium-parser.ts

@@ -233,13 +233,15 @@ export class LangiumParser extends AbstractLangiumParser {
         if (assignment) {
             this.assign(assignment.operator, assignment.feature, result, cstNode, isCrossRef);
         } else if (!assignment) {
-            // If we call a subrule without an assignment
-            // We either append the result of the subrule (data type rule)
-            // Or override the current object with the newly parsed object
+            // If we call a subrule without an assignment we either:
+            // 1. append the result of the subrule (data type rule)
+            // 2. override the current object with the newly parsed object
+            // If the current element is an AST node and the result of the subrule
+            // is a data type rule, we can safely discard the results.
             const current = this.current;
             if (isDataTypeNode(current)) {
                 current.value += result.toString();
-            } else {
+            } else if (typeof result === 'object' && result) {
                 const resultKind = result.$type;
                 const object = this.assignWithoutOverride(result, current);
                 if (resultKind) {

packages/langium/test/parser/langium-parser-builder.test.ts

@@ -9,6 +9,7 @@ import type { AstNode, CstNode, GenericAstNode, Grammar, GrammarAST, LangiumPars
 import { EmptyFileSystem, DefaultTokenBuilder } from 'langium';
 import { describe, expect, test, onTestFailed, beforeEach } from 'vitest';
 import { createLangiumGrammarServices, createServicesForGrammar } from 'langium/grammar';
+import { expandToString } from 'langium/generate';
 import { parseHelper } from 'langium/test';
 import { EOF } from 'chevrotain';
 
@@ -826,6 +827,34 @@ describe('Handling EOF', () => {
     }
 });
 
+describe('Unassigned data type rules', () => {
+
+    test('Can successfully parse unassigned data type rule in parser rule', async () => {
+        const parser = await parserFromGrammar(expandToString`
+            grammar HelloWorld
+
+            entry Model:
+                ((persons+=Person) EOL)*;
+
+            Person:
+                'person' name=ID;
+
+            hidden terminal WS: /[ \\t]+/;
+            terminal ID: /[_a-zA-Z][\\w_]*/;
+            terminal NEWLINE: /\\r?\\n/;
+            EOL returns string:
+                NEWLINE | EOF;`
+        );
+        const parseResult = parser.parse(expandToString`
+            person John
+            person Jane
+        `);
+        expect(parseResult.lexerErrors).toHaveLength(0);
+        expect(parseResult.parserErrors).toHaveLength(0);
+    });
+
+});
+
 async function parserFromGrammar(grammar: string): Promise<LangiumParser> {
     return (await createServicesForGrammar({ grammar })).parser.LangiumParser;
 }