gh-145239: Specialize match syntax error for unary addition in pattern

Add a specialized syntax error message when unary '+' is used in match
patterns. Instead of the generic "invalid syntax", users now see:
"cannot use unary '+' in a literal pattern".

This applies to all pattern contexts: top-level, alternatives, sequences,
class patterns, and mapping patterns.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mvanhorn

Grammar/python.gram

@@ -536,6 +536,7 @@ literal_pattern[pattern_ty]:
     | 'None' { _PyAST_MatchSingleton(Py_None, EXTRA) }
     | 'True' { _PyAST_MatchSingleton(Py_True, EXTRA) }
     | 'False' { _PyAST_MatchSingleton(Py_False, EXTRA) }
+    | invalid_literal_pattern
 
 # Literal expressions are used to restrict permitted mapping pattern keys
 literal_expr[expr_ty]:
@@ -1519,6 +1520,8 @@ invalid_mapping_pattern:
         "double star pattern must be the last (right-most) subpattern in the mapping pattern") }
 invalid_class_argument_pattern[asdl_pattern_seq*]:
     | [positional_patterns ','] keyword_patterns ',' a=positional_patterns { a }
+invalid_literal_pattern:
+    | a='+' NUMBER { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot use unary '+' in a literal pattern") }
 invalid_if_stmt:
     | 'if' named_expression NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
     | a='if' a=named_expression ':' NEWLINE !INDENT {

Lib/test/test_patma.py

@@ -3230,6 +3230,41 @@ def test_mapping_pattern_duplicate_key_edge_case3(self):
                 pass
         """)
 
+    def test_unary_add_in_literal_pattern(self):
+        self.assert_syntax_error("""
+        match ...:
+            case +1:
+                pass
+        """)
+
+    def test_unary_add_in_or_pattern(self):
+        self.assert_syntax_error("""
+        match ...:
+            case 1 | +2 | -3:
+                pass
+        """)
+
+    def test_unary_add_in_sequence_pattern(self):
+        self.assert_syntax_error("""
+        match ...:
+            case [1, +2, -3]:
+                pass
+        """)
+
+    def test_unary_add_in_class_pattern(self):
+        self.assert_syntax_error("""
+        match ...:
+            case Foo(x=+1, y=-2):
+                pass
+        """)
+
+    def test_unary_add_in_mapping_pattern(self):
+        self.assert_syntax_error("""
+        match ...:
+            case {True: +1, False: -2}:
+                pass
+        """)
+
 class TestTypeErrors(unittest.TestCase):
 
     def test_accepts_positional_subpatterns_0(self):

Lib/test/test_syntax.py

@@ -2399,6 +2399,38 @@
     Traceback (most recent call last):
     SyntaxError: double star pattern must be the last (right-most) subpattern in the mapping pattern
 
+Unary '+' is not allowed in match patterns:
+
+    >>> match ...:
+    ...   case +1:
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: cannot use unary '+' in a literal pattern
+
+    >>> match ...:
+    ...   case 1 | +2 | -3:
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: cannot use unary '+' in a literal pattern
+
+    >>> match ...:
+    ...   case [1, +2, -3]:
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: cannot use unary '+' in a literal pattern
+
+    >>> match ...:
+    ...   case Foo(x=+1, y=-2):
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: cannot use unary '+' in a literal pattern
+
+    >>> match ...:
+    ...   case {True: +1, False: -2}:
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: cannot use unary '+' in a literal pattern
+
 Uses of the star operator which should fail:
 
 A[:*b]

Misc/NEWS.d/next/Core_and_Builtins/2026-03-08-00-00-00.gh-issue-145239.UnaryAdd.rst

@@ -0,0 +1,3 @@
+Improved the error message when using unary ``+`` in a :keyword:`match`
+pattern. Instead of a generic "invalid syntax", Python now reports "cannot
+use unary '+' in a literal pattern".