Narrowing for comparisons against x.__class__ (#20642)

Author: hauntsaninja

mypy/checker.py

@@ -6734,6 +6734,8 @@ def narrow_type_by_identity_equality(
                 and len(type_expr.args) == 1
             ):
                 expr_in_type_expr = type_expr.args[0]
+            elif isinstance(type_expr, MemberExpr) and type_expr.name == "__class__":
+                expr_in_type_expr = type_expr.expr
             else:
                 continue
             for j in expr_indices:

test-data/unit/check-narrowing.test

@@ -3136,3 +3136,31 @@ if type(x) is not C:
     reveal_type(x)  # N: Revealed type is "__main__.D"
 else:
     reveal_type(x)  # N: Revealed type is "__main__.C"
+
+[case testDunderClassNarrowing]
+# flags: --warn-unreachable
+from typing import Any
+
+def foo(y: object):
+    if y.__class__ == int:
+        reveal_type(y) # N: Revealed type is "builtins.int"
+    else:
+        reveal_type(y) # N: Revealed type is "builtins.object"
+
+    if y.__class__ is int:
+        reveal_type(y) # N: Revealed type is "builtins.int"
+    else:
+        reveal_type(y) # N: Revealed type is "builtins.object"
+
+
+def bar(y: Any):
+    if y.__class__ == int:
+        reveal_type(y) # N: Revealed type is "Any"
+    else:
+        reveal_type(y) # N: Revealed type is "Any"
+
+    if y.__class__ is int:
+        reveal_type(y) # N: Revealed type is "builtins.int"
+    else:
+        reveal_type(y) # N: Revealed type is "Any"
+[builtins fixtures/dict-full.pyi]

test-data/unit/fixtures/dict-full.pyi

@@ -13,6 +13,7 @@ KT = TypeVar('KT')
 VT = TypeVar('VT')
 
 class object:
+    __class__: object
     def __init__(self) -> None: pass
     def __init_subclass__(cls) -> None: pass
     def __eq__(self, other: object) -> bool: pass
@@ -75,8 +76,7 @@ class float: pass
 class complex: pass
 class bool(int): pass
 
-class ellipsis:
-    __class__: object
+class ellipsis: pass
 def isinstance(x: object, t: Union[type, Tuple[type, ...]]) -> bool: pass
 class BaseException: pass