Allow void type in generics parameter default type

Author: soutaro

src/parser.c

@@ -1438,7 +1438,7 @@ static bool parse_type_params(rbs_parser_t *parser, rbs_range_t *rg, bool module
                     rbs_parser_advance(parser);
 
                     default_type_range.start = parser->current_token.range.start;
-                    CHECK_PARSE(rbs_parse_type(parser, &default_type, false));
+                    CHECK_PARSE(rbs_parse_type(parser, &default_type, true));
                     default_type_range.end = parser->current_token.range.end;
 
                     required_param_allowed = false;

test/rbs/signature_parsing_test.rb

@@ -2370,4 +2370,11 @@ def return: (untyped return) -> void
       end
     RBS
   end
+
+  def test_generics__default_type_void
+    Parser.parse_signature(<<~RBS)
+      class Foo[T = void]
+      end
+    RBS
+  end
 end