gh-154775: Don't accept duplicate plus sign when matching complex number

Author: encukou

Grammar/python.gram

@@ -568,7 +568,6 @@ real_number[expr_ty]:
 
 imaginary_number[expr_ty]:
     | imag=NUMBER { _PyPegen_ensure_imaginary(p, imag) }
-    | '+' imag=NUMBER { _PyPegen_ensure_imaginary(p, imag) }
 
 capture_pattern[pattern_ty]:
     | target=pattern_capture_target { _PyAST_MatchAs(NULL, target->v.Name.id, EXTRA) }

Lib/test/test_patma.py

@@ -3329,6 +3329,34 @@ def test_mapping_pattern_duplicate_key_edge_case3(self):
                 pass
         """)
 
+    def test_duplicate_sign_in_complex_1(self):
+        self.assert_syntax_error("""
+        match ...:
+            case 0 ++ 0j:
+                pass
+        """)
+
+    def test_duplicate_sign_in_complex_2(self):
+        self.assert_syntax_error("""
+        match ...:
+            case 0 -+ 0j:
+                pass
+        """)
+
+    def test_duplicate_sign_in_complex_3(self):
+        self.assert_syntax_error("""
+        match ...:
+            case 0 +- 0j:
+                pass
+        """)
+
+    def test_duplicate_sign_in_complex_4(self):
+        self.assert_syntax_error("""
+        match ...:
+            case 0 -- 0j:
+                pass
+        """)
+
 class TestTypeErrors(unittest.TestCase):
 
     def test_accepts_positional_subpatterns_0(self):

Misc/NEWS.d/next/Core_and_Builtins/2026-07-27-16-29-10.gh-issue-154775._ISRIk.rst

@@ -0,0 +1,2 @@
+When matching a complex literal in :keyword:`case` statements, an extraneous
+``+`` sign (for example, ``1++1j`` or ``1-+1j``) is no longer allowed.