bpo-44368: Improve syntax errors with invalid as pattern targets (GH-26632)

Author: pablogsal

Grammar/python.gram

@@ -244,6 +244,7 @@ pattern[pattern_ty]:
 as_pattern[pattern_ty]:
     | pattern=or_pattern 'as' target=pattern_capture_target {
         _PyAST_MatchAs(pattern, target->v.Name.id, EXTRA) }
+    | invalid_as_pattern
 or_pattern[pattern_ty]:
     | patterns[asdl_pattern_seq*]='|'.closed_pattern+ {
         asdl_seq_LEN(patterns) == 1 ? asdl_seq_GET(patterns, 0) : _PyAST_MatchOr(patterns, EXTRA) }
@@ -974,6 +975,9 @@ invalid_case_block:
     | "case" patterns guard? !':' { RAISE_SYNTAX_ERROR("expected ':'") }
     | a="case" patterns guard? ':' NEWLINE !INDENT {
         RAISE_INDENTATION_ERROR("expected an indented block after 'case' statement on line %d", a->lineno) }
+invalid_as_pattern:
+    | or_pattern 'as' a="_" { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot use '_' as a target") }
+    | or_pattern 'as' !NAME a=expression { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "invalid pattern target") }
 invalid_if_stmt:
     | 'if' named_expression NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
     | a='if' a=named_expression ':' NEWLINE !INDENT {

Lib/test/test_syntax.py

@@ -1226,6 +1226,20 @@
     >>> import ä £
     Traceback (most recent call last):
     SyntaxError: invalid character '£' (U+00A3)
+
+  Invalid pattern matching constructs:
+
+    >>> match ...:
+    ...   case 42 as _:
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: cannot use '_' as a target
+
+    >>> match ...:
+    ...   case 42 as 1+2+4:
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: invalid pattern target
 """
 
 import re

Misc/NEWS.d/next/Core and Builtins/2021-06-09-22-56-59.bpo-44368.vgT0Cx.rst

@@ -0,0 +1 @@
+Improve syntax errors for invalid "as" targets. Patch by Pablo Galindo