gh-118090: Improve error message for empty type param brackets (GH-118091)

Grammar/python.gram

@@ -269,11 +269,11 @@ function_def[stmt_ty]:
 
 function_def_raw[stmt_ty]:
     | invalid_def_raw
-    | 'def' n=NAME t=[type_params] &&'(' params=[params] ')' a=['->' z=expression { z }] &&':' tc=[func_type_comment] b=block {
+    | 'def' n=NAME t=[type_params] '(' params=[params] ')' a=['->' z=expression { z }] ':' tc=[func_type_comment] b=block {
         _PyAST_FunctionDef(n->v.Name.id,
                         (params) ? params : CHECK(arguments_ty, _PyPegen_empty_arguments(p)),
                         b, NULL, a, NEW_TYPE_COMMENT(p, tc), t, EXTRA) }
-    | 'async' 'def' n=NAME t=[type_params] &&'(' params=[params] ')' a=['->' z=expression { z }] &&':' tc=[func_type_comment] b=block {
+    | 'async' 'def' n=NAME t=[type_params] '(' params=[params] ')' a=['->' z=expression { z }] ':' tc=[func_type_comment] b=block {
         CHECK_VERSION(
             stmt_ty,
             5,
@@ -641,7 +641,9 @@ type_alias[stmt_ty]:
 # Type parameter declaration
 # --------------------------
 
-type_params[asdl_type_param_seq*]: '[' t=type_param_seq  ']' {
+type_params[asdl_type_param_seq*]: 
+    | invalid_type_params
+    | '[' t=type_param_seq ']' {
         CHECK_VERSION(asdl_type_param_seq *, 12, "Type parameter lists are", t) }
 
 type_param_seq[asdl_type_param_seq*]: a[asdl_type_param_seq*]=','.type_param+ [','] { a }
@@ -1392,6 +1394,7 @@ invalid_for_stmt:
 invalid_def_raw:
     | ['async'] a='def' NAME [type_params] '(' [params] ')' ['->' expression] ':' NEWLINE !INDENT {
         RAISE_INDENTATION_ERROR("expected an indented block after function definition on line %d", a->lineno) }
+    | ['async'] 'def' NAME [type_params] &&'(' [params] ')' ['->' expression] &&':' [func_type_comment] block
 invalid_class_def_raw:
     | 'class' NAME [type_params] ['(' [arguments] ')'] NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
     | a='class' NAME [type_params] ['(' [arguments] ')'] ':' NEWLINE !INDENT {
@@ -1435,3 +1438,9 @@ invalid_arithmetic:
     | sum ('+'|'-'|'*'|'/'|'%'|'//'|'@') a='not' b=inversion { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "'not' after an operator must be parenthesized") }
 invalid_factor:
     | ('+' | '-' | '~') a='not' b=factor { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "'not' after an operator must be parenthesized") }
+
+invalid_type_params:
+    | '[' token=']' {
+        RAISE_SYNTAX_ERROR_STARTING_FROM(
+            token, 
+            "Type parameter list cannot be empty")}

Lib/test/test_syntax.py

@@ -1213,6 +1213,22 @@ Missing parens after function definition
    Traceback (most recent call last):
    SyntaxError: expected '('
 
+   >>> def f -> int:
+   Traceback (most recent call last):
+   SyntaxError: expected '('
+
+   >>> async def f -> int:  # type: int
+   Traceback (most recent call last):
+   SyntaxError: expected '('
+
+   >>> async def f[T]:
+   Traceback (most recent call last):
+   SyntaxError: expected '('
+
+   >>> def f[T] -> str:
+   Traceback (most recent call last):
+   SyntaxError: expected '('
+
 Parenthesized arguments in function definitions
 
    >>> def f(x, (y, z), w):
@@ -2027,6 +2043,31 @@ Invalid bytes literals:
 
 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
+
+   >>> def some[]()
+   Traceback (most recent call last):
+   ...
+   SyntaxError: Type parameter list cannot be empty
+
+   >>> async def some[]:  # type: int
+   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.