[3.15] gh-154775: Don't accept duplicate plus sign when matching complex number (GH-154776) (GH-154802)

(cherry picked from commit a85830c)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Bartosz Sławecki <bartosz@ilikepython.com>

Author: 3 people

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

@@ -2835,14 +2835,6 @@ def test_patma_264(self):
         self.assertEqual(y, 0)
 
     def test_patma_265(self):
-        x = 0.25 - 1.75j
-        match x:
-            case 0.25 - +1.75j:
-                y = 0
-        self.assertEqual(x, 0.25 - 1.75j)
-        self.assertEqual(y, 0)
-
-    def test_patma_266(self):
         x = 0
         match x:
             case +1e1000:
@@ -3329,6 +3321,34 @@ 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.