3 files changed, 106 insertions, 0 deletions
diff --git a/Lib/test/test_peg_generator/test_grammar_validator.py b/Lib/test/test_peg_generator/test_grammar_validator.py
new file mode 100644
index 0000000..2e72ff8
--- /dev/null
+++ b/Lib/test/test_peg_generator/test_grammar_validator.py
@@ -0,0 +1,51 @@
+import unittest
+from test import test_tools
+
+test_tools.skip_if_missing('peg_generator')
+with test_tools.imports_under_tool('peg_generator'):
+    from pegen.grammar_parser import GeneratedParser as GrammarParser
+    from pegen.validator import SubRuleValidator, ValidationError
+    from pegen.testutil import parse_string
+    from pegen.grammar import Grammar
+
+
+class TestPegen(unittest.TestCase):
+    def test_rule_with_no_collision(self) -> None:
+        grammar_source = """
+        start: bad_rule
+        sum:
+            | NAME '-' NAME
+            | NAME '+' NAME
+        """
+        grammar: Grammar = parse_string(grammar_source, GrammarParser)
+        validator = SubRuleValidator(grammar)
+        for rule_name, rule in grammar.rules.items():
+            validator.validate_rule(rule_name, rule)
+
+    def test_rule_with_simple_collision(self) -> None:
+        grammar_source = """
+        start: bad_rule
+        sum:
+            | NAME '+' NAME
+            | NAME '+' NAME ';'
+        """
+        grammar: Grammar = parse_string(grammar_source, GrammarParser)
+        validator = SubRuleValidator(grammar)
+        with self.assertRaises(ValidationError):
+            for rule_name, rule in grammar.rules.items():
+                validator.validate_rule(rule_name, rule)
+
+    def test_rule_with_collision_after_some_other_rules(self) -> None:
+        grammar_source = """
+        start: bad_rule
+        sum:
+            | NAME '+' NAME
+            | NAME '*' NAME ';'
+            | NAME '-' NAME
+            | NAME '+' NAME ';'
+        """
+        grammar: Grammar = parse_string(grammar_source, GrammarParser)
+        validator = SubRuleValidator(grammar)
+        with self.assertRaises(ValidationError):
+            for rule_name, rule in grammar.rules.items():
+                validator.validate_rule(rule_name, rule)
diff --git a/Tools/peg_generator/pegen/__main__.py b/Tools/peg_generator/pegen/__main__.py
index 1dcbaad..c0f3b68 100755
--- a/Tools/peg_generator/pegen/__main__.py
+++ b/Tools/peg_generator/pegen/__main__.py
@@ -14,6 +14,7 @@ import traceback
 from typing import Tuple
 
 from pegen.build import Grammar, Parser, Tokenizer, ParserGenerator
+from pegen.validator import validate_grammar
 
 
 def generate_c_code(
@@ -128,6 +129,8 @@ def main() -> None:
     grammar, parser, tokenizer, gen = args.func(args)
     t1 = time.time()
 
+    validate_grammar(grammar)
+
     if not args.quiet:
         if args.verbose:
             print("Raw Grammar:")
diff --git a/Tools/peg_generator/pegen/validator.py b/Tools/peg_generator/pegen/validator.py
new file mode 100644
index 0000000..0e3dd41
--- /dev/null
+++ b/Tools/peg_generator/pegen/validator.py
@@ -0,0 +1,52 @@
+from pegen import grammar
+from pegen.grammar import (
+    Alt,
+    Cut,
+    Gather,
+    GrammarVisitor,
+    Group,
+    Lookahead,
+    NamedItem,
+    NameLeaf,
+    NegativeLookahead,
+    Opt,
+    PositiveLookahead,
+    Repeat0,
+    Repeat1,
+    Rhs,
+    Rule,
+    StringLeaf,
+)
+
+class ValidationError(Exception):
+    pass
+
+class GrammarValidator(GrammarVisitor):
+    def __init__(self, grammar: grammar.Grammar):
+        self.grammar = grammar
+        self.rulename = None
+
+    def validate_rule(self, rulename: str, node: Rule):
+        self.rulename = rulename
+        self.visit(node)
+        self.rulename = None
+
+
+class SubRuleValidator(GrammarValidator):
+    def visit_Rhs(self, node: Rule):
+        for index, alt in enumerate(node.alts):
+            alts_to_consider = node.alts[index+1:]
+            for other_alt in alts_to_consider:
+                self.check_intersection(alt, other_alt)
+
+    def check_intersection(self, first_alt: Alt, second_alt: Alt) -> bool:
+        if str(second_alt).startswith(str(first_alt)):
+            raise ValidationError(
+                    f"In {self.rulename} there is an alternative that will "
+                    f"never be visited:\n{second_alt}")
+
+def validate_grammar(the_grammar: grammar.Grammar):
+    for validator_cls in GrammarValidator.__subclasses__():
+        validator = validator_cls(the_grammar)
+        for rule_name, rule in the_grammar.rules.items():
+            validator.validate_rule(rule_name, rule)