gh-118090: Improve error message for empty type param brackets

Grammar/python.gram

@@ -638,8 +638,13 @@ type_alias[stmt_ty]:
 # Type parameter declaration
 # --------------------------
 
-type_params[asdl_type_param_seq*]: '[' t=type_param_seq  ']' {
+type_params[asdl_type_param_seq*]: 
+    | '[' t=type_param_seq  ']' {
         CHECK_VERSION(asdl_type_param_seq *, 12, "Type parameter lists are", t) }
+    | '[' token=']' {
+        RAISE_SYNTAX_ERROR_STARTING_FROM(
+            token, 
+            "Type parameter list cannot be empty")}
 
 type_param_seq[asdl_type_param_seq*]: a[asdl_type_param_seq*]=','.type_param+ [','] { a }
 

Lib/test/test_syntax.py

@@ -2015,6 +2015,21 @@ def f(x: *b)
 
 Invalid expressions in type scopes:
 
+   >>> type A[] = int
+   Traceback (most recent call last):
+   ...
+   SyntaxError: Type parameter list cannot be empty
+
+   >>> class A[]: ...
+   Traceback (most recent call last):
+   ...
+   SyntaxError: Type parameter list cannot be empty
+
+   >>> def some[](): ...
+   Traceback (most recent call last):
+   ...
+   SyntaxError: Type parameter list cannot be empty
+
    >>> type A[T: (x:=3)] = int
    Traceback (most recent call last):
       ...

Misc/NEWS.d/next/Core and Builtins/2024-04-19-11-57-46.gh-issue-118090.eGAQ0B.rst

@@ -0,0 +1 @@
+Improve :exc:`SyntaxError` message for empty type param brackets.