From 9cdfb193831dade20c56e29220d78f7dc1d68718 Mon Sep 17 00:00:00 2001
From: A5rocks <git@helvetica.moe>
Date: Fri, 31 Jan 2025 08:54:04 +0900
Subject: [PATCH] Narrow literals in the negative case even with custom
 equality

---
 mypy/checker.py                     | 43 +++++++++++++++++------------
 test-data/unit/check-narrowing.test | 26 ++++++++++++++++-
 2 files changed, 51 insertions(+), 18 deletions(-)

diff --git a/mypy/checker.py b/mypy/checker.py
index c69b80a55fd9c..de0e028b32754 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -6413,33 +6413,42 @@ def equality_type_narrowing_helper(
             should_narrow_by_identity = True
         else:
 
-            def is_exactly_literal_type(t: Type) -> bool:
-                return isinstance(get_proper_type(t), LiteralType)
-
             def has_no_custom_eq_checks(t: Type) -> bool:
                 return not custom_special_method(
                     t, "__eq__", check_all=False
                 ) and not custom_special_method(t, "__ne__", check_all=False)
 
-            is_valid_target = is_exactly_literal_type
-            coerce_only_in_literal_context = True
-
             expr_types = [operand_types[i] for i in expr_indices]
             should_narrow_by_identity = all(
                 map(has_no_custom_eq_checks, expr_types)
             ) and not is_ambiguous_mix_of_enums(expr_types)
 
-        if_map: TypeMap = {}
-        else_map: TypeMap = {}
-        if should_narrow_by_identity:
-            if_map, else_map = self.refine_identity_comparison_expression(
-                operands,
-                operand_types,
-                expr_indices,
-                narrowable_operand_index_to_hash.keys(),
-                is_valid_target,
-                coerce_only_in_literal_context,
-            )
+            def is_exactly_literal_type_possibly_except_enum(t: Type) -> bool:
+                p_t = get_proper_type(t)
+                if isinstance(p_t, LiteralType):
+                    if should_narrow_by_identity:
+                        return True
+                    else:
+                        return not p_t.fallback.type.is_enum
+                else:
+                    return False
+
+            is_valid_target = is_exactly_literal_type_possibly_except_enum
+            coerce_only_in_literal_context = True
+
+        if_map, else_map = self.refine_identity_comparison_expression(
+            operands,
+            operand_types,
+            expr_indices,
+            narrowable_operand_index_to_hash.keys(),
+            is_valid_target,
+            coerce_only_in_literal_context,
+        )
+        if not should_narrow_by_identity:
+            # refine_identity_comparison_expression narrows against a single literal
+            # -- and we know that literal will only go to the positive branch.
+            # This means that the negative branch narrowing is actually correct.
+            if_map = {}
 
         if if_map == {} and else_map == {}:
             if_map, else_map = self.refine_away_none_in_comparison(
diff --git a/test-data/unit/check-narrowing.test b/test-data/unit/check-narrowing.test
index ec647366e7437..835b416270fc0 100644
--- a/test-data/unit/check-narrowing.test
+++ b/test-data/unit/check-narrowing.test
@@ -845,7 +845,7 @@ x1: Union[Custom, Literal[1], Literal[2]]
 if x1 == 1:
     reveal_type(x1)  # N: Revealed type is "Union[__main__.Custom, Literal[1], Literal[2]]"
 else:
-    reveal_type(x1)  # N: Revealed type is "Union[__main__.Custom, Literal[1], Literal[2]]"
+    reveal_type(x1)  # N: Revealed type is "Union[__main__.Custom, Literal[2]]"
 
 x2: Union[Default, Literal[1], Literal[2]]
 if x2 == 1:
@@ -2416,3 +2416,27 @@ while x is not None and b():
     x = f()
 
 [builtins fixtures/primitives.pyi]
+
+[case testNegativeNarrowingWithCustomEq]
+from typing import Union
+from typing_extensions import Literal
+
+class A:
+    def __eq__(self, other: object) -> bool: ...  # necessary
+
+def f(v: Union[A, Literal["text"]]) -> Union[A, None]:
+    if v == "text":
+        reveal_type(v)  # N: Revealed type is "Union[__main__.A, Literal['text']]"
+        return None
+    else:
+        reveal_type(v)  # N: Revealed type is "__main__.A"
+        return v  # no error
+
+def g(v: Union[A, Literal["text"]]) -> Union[A, None]:
+    if v != "text":
+        reveal_type(v)  # N: Revealed type is "__main__.A"
+        return None
+    else:
+        reveal_type(v)  # N: Revealed type is "Union[__main__.A, Literal['text']]"
+        return v  # E: Incompatible return value type (got "Union[A, Literal['text']]", expected "Optional[A]")
+[builtins fixtures/primitives.pyi]