Ensure that empty lexer alternatives are handled properly

Fixes #50.

Author: renatahodovan

grammarinator/process.py

@@ -428,7 +428,12 @@ def build_expr(node, parent_id):
                     graph.add_edge(frm=parent_id, to=lambda_id)
                     return
 
-                for child in node.element() if isinstance(node, ANTLRv4Parser.AlternativeContext) else node.lexerElements().lexerElement():
+                children = node.element() if isinstance(node, ANTLRv4Parser.AlternativeContext) else node.lexerElements().lexerElement()
+                if isinstance(node, ANTLRv4Parser.LexerAltContext) and not children:
+                    graph.add_edge(frm=parent_id, to=lambda_id)
+                    return
+
+                for child in children:
                     build_expr(child, parent_id)
 
             elif isinstance(node, (ANTLRv4Parser.ElementContext, ANTLRv4Parser.LexerElementContext)):

tests/grammars/EmptyAlternatives.g4

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2022 Renata Hodovan, Akos Kiss.
+ *
+ * Licensed under the BSD 3-Clause License
+ * <LICENSE.rst or https://opensource.org/licenses/BSD-3-Clause>.
+ * This file may not be copied, modified, or distributed except
+ * according to those terms.
+ */
+
+/*
+ * This test checks whether the empty lexer and parser alternatives
+ * are handled properly.
+ */
+
+// TEST-PROCESS: {grammar}.g4 -o {tmpdir}
+// TEST-GENERATE: {grammar}Generator.{grammar}Generator -r start -o {tmpdir}/{grammar}%d.txt
+// TEST-ANTLR: {grammar}.g4 -o {tmpdir}
+// TEST-PARSE: -p {grammar}Parser -l {grammar}Lexer -r start {tmpdir}/{grammar}%d.txt
+
+grammar EmptyAlternatives;
+
+start: | EMPTY ;
+
+EMPTY : 'a' ( | 'b') ;