Disallow "true", "false", and "null" as enum values

Replicates graphql/graphql-js@af878f3

Author: Cito

src/graphql/language/parser.py

@@ -820,7 +820,7 @@ def parse_enum_value_definition(self) -> EnumValueDefinitionNode:
         """EnumValueDefinition: Description? EnumValue Directives[Const]?"""
         start = self._lexer.token
         description = self.parse_description()
-        name = self.parse_name()
+        name = self.parse_enum_value_name()
         directives = self.parse_const_directives()
         return EnumValueDefinitionNode(
             description=description,
@@ -829,6 +829,17 @@ def parse_enum_value_definition(self) -> EnumValueDefinitionNode:
             loc=self.loc(start),
         )
 
+    def parse_enum_value_name(self) -> NameNode:
+        """EnumValue: Name but not ``true``, ``false`` or ``null``"""
+        if self._lexer.token.value in ("true", "false", "null"):
+            raise GraphQLSyntaxError(
+                self._lexer.source,
+                self._lexer.token.start,
+                f"{get_token_desc(self._lexer.token)} is reserved"
+                " and cannot be used for an enum value.",
+            )
+        return self.parse_name()
+
     def parse_input_object_type_definition(self) -> InputObjectTypeDefinitionNode:
         """InputObjectTypeDefinition"""
         start = self._lexer.token

tests/language/test_parser.py

@@ -112,6 +112,23 @@ def does_not_accept_fragments_named_on():
     def does_not_accept_fragments_spread_of_on():
         assert_syntax_error("{ ...on }", "Expected Name, found '}'.", (1, 9))
 
+    def does_not_allow_true_false_or_null_as_enum_value():
+        assert_syntax_error(
+            "enum Test { VALID, true }",
+            "Name 'true' is reserved and cannot be used for an enum value.",
+            (1, 20),
+        )
+        assert_syntax_error(
+            "enum Test { VALID, false }",
+            "Name 'false' is reserved and cannot be used for an enum value.",
+            (1, 20),
+        )
+        assert_syntax_error(
+            "enum Test { VALID, null }",
+            "Name 'null' is reserved and cannot be used for an enum value.",
+            (1, 20),
+        )
+
     def parses_multi_byte_characters():
         # Note: \u0A0A could be naively interpreted as two line-feed chars.
         doc = parse(