Python: Add grammar support for type defaults

Also fixes an oversight in the grammar: starred expressions should be
allowed inside the subscript of an `Index` expression.

Author: tausbn

python/extractor/tsg-python/tsp/grammar.js

@@ -589,17 +589,20 @@ module.exports = grammar({
 
     typevar_parameter: $ => seq(
       field('name', $.identifier),
-      optional($._type_bound)
+      optional($._type_bound),
+      optional($._type_param_default)
     ),
 
     typevartuple_parameter: $ => seq(
       '*',
       field('name', $.identifier),
+      optional($._type_param_default)
     ),
 
     paramspec_parameter: $ => seq(
       '**',
       field('name', $.identifier),
+      optional($._type_param_default),
     ),
 
     _type_parameter: $ => choice(
@@ -608,6 +611,11 @@ module.exports = grammar({
         $.paramspec_parameter,
     ),
 
+    _type_param_default: $ => seq(
+      '=',
+      field('default', choice($.list_splat, $.expression))
+    ),
+
     parenthesized_list_splat: $ => prec(PREC.parenthesized_list_splat, seq(
       '(',
       choice(
@@ -923,7 +931,7 @@ module.exports = grammar({
     subscript: $ => prec(PREC.call, seq(
       field('value', $.primary_expression),
       '[',
-      commaSep1(field('subscript', choice($.expression, $.slice))),
+      commaSep1(field('subscript', choice($.list_splat, $.expression, $.slice))),
       optional(','),
       ']'
     )),