Rebind self-types in subclass methods without Self annotation (#15541)

Fixes #15529

The fix is straightforward, hopefully there will be no fallout. (Note
that #14075 would also fix this, but I am still not sure we should do
that)

---------

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

mypy/checkmember.py

@@ -735,12 +735,12 @@ def analyze_var(
     """Analyze access to an attribute via a Var node.
 
     This is conceptually part of analyze_member_access and the arguments are similar.
-
-    itype is the class object in which var is defined
+    itype is the instance type in which attribute should be looked up
     original_type is the type of E in the expression E.var
     if implicit is True, the original Var was created as an assignment to self
     """
     # Found a member variable.
+    original_itype = itype
     itype = map_instance_to_supertype(itype, var.info)
     typ = var.type
     if typ:
@@ -756,6 +756,16 @@ def analyze_var(
             get_proper_type(mx.original_type)
         ):
             t = expand_self_type(var, t, mx.original_type)
+        elif (
+            mx.is_self
+            and original_itype.type != var.info
+            # If an attribute with Self-type was defined in a supertype, we need to
+            # rebind the Self type variable to Self type variable of current class...
+            and original_itype.type.self_type is not None
+            # ...unless `self` has an explicit non-trivial annotation.
+            and original_itype == mx.chk.scope.active_self_type()
+        ):
+            t = expand_self_type(var, t, original_itype.type.self_type)
         t = get_proper_type(expand_type_by_instance(t, itype))
         freeze_all_type_vars(t)
         result: Type = t

mypy/plugins/dataclasses.py

@@ -355,7 +355,8 @@ def transform(self) -> bool:
         self._add_dataclass_fields_magic_attribute()
 
         if self._spec is _TRANSFORM_SPEC_FOR_DATACLASSES:
-            self._add_internal_replace_method(attributes)
+            with state.strict_optional_set(self._api.options.strict_optional):
+                self._add_internal_replace_method(attributes)
         if "__post_init__" in info.names:
             self._add_internal_post_init_method(attributes)
 

mypy/semanal.py

@@ -1692,6 +1692,8 @@ def is_core_builtin_class(self, defn: ClassDef) -> bool:
     def analyze_class_body_common(self, defn: ClassDef) -> None:
         """Parts of class body analysis that are common to all kinds of class defs."""
         self.enter_class(defn.info)
+        if any(b.self_type is not None for b in defn.info.mro):
+            self.setup_self_type()
         defn.defs.accept(self)
         self.apply_class_plugin_hooks(defn)
         self.leave_class()

test-data/unit/check-selftype.test

@@ -1867,3 +1867,20 @@ class B:
             return B()  # E: Incompatible return value type (got "B", expected "A")
 
 [builtins fixtures/isinstancelist.pyi]
+
+[case testAttributeOnSelfAttributeInSubclass]
+from typing import List, Self
+
+class A:
+    x: Self
+    xs: List[Self]
+
+class B(A):
+    extra: int
+
+    def meth(self) -> None:
+        reveal_type(self.x)  # N: Revealed type is "Self`0"
+        reveal_type(self.xs[0])  # N: Revealed type is "Self`0"
+        reveal_type(self.x.extra)  # N: Revealed type is "builtins.int"
+        reveal_type(self.xs[0].extra)  # N: Revealed type is "builtins.int"
+[builtins fixtures/list.pyi]

test-data/unit/pythoneval.test

@@ -2092,3 +2092,20 @@ def fst(kv: Tuple[K, V]) -> K:
 pairs = [(len(s), s) for s in ["one", "two", "three"]]
 grouped = groupby(pairs, key=fst)
 [out]
+
+[case testDataclassReplaceOptional]
+# flags: --strict-optional
+from dataclasses import dataclass, replace
+from typing import Optional
+
+@dataclass
+class A:
+    x: Optional[int]
+
+a = A(x=42)
+reveal_type(a)
+a2 = replace(a, x=None)  # OK
+reveal_type(a2)
+[out]
+_testDataclassReplaceOptional.py:10: note: Revealed type is "_testDataclassReplaceOptional.A"
+_testDataclassReplaceOptional.py:12: note: Revealed type is "_testDataclassReplaceOptional.A"