Handle union types when binding self (python#18867)

Currently we only bind self if the type is callable, but we actually
should do this for all callable items in a union.

This use case is probably quite niche (since adding an annotation makes
a variable an instance variable, and we rarely infer unions). I found it
when looking at `checkmember`-related issues it was easy to handle it. I
also use this opportunity to refactor and add comments to
`analyze_var()`.

Author: ilevkivskyi

mypy/checkmember.py

@@ -870,15 +870,13 @@ def analyze_var(
                 mx.msg.read_only_property(name, itype.type, mx.context)
             if var.is_classvar:
                 mx.msg.cant_assign_to_classvar(name, mx.context)
-        t = freshen_all_functions_type_vars(typ)
-        t = expand_self_type_if_needed(t, mx, var, original_itype)
-        t = expand_type_by_instance(t, itype)
-        freeze_all_type_vars(t)
-        result = t
-        typ = get_proper_type(typ)
+        # This is the most common case for variables, so start with this.
+        result = expand_without_binding(typ, var, itype, original_itype, mx)
 
+        # A non-None value indicates that we should actually bind self for this variable.
         call_type: ProperType | None = None
         if var.is_initialized_in_class and (not is_instance_var(var) or mx.is_operator):
+            typ = get_proper_type(typ)
             if isinstance(typ, FunctionLike) and not typ.is_type_obj():
                 call_type = typ
             elif var.is_property:
@@ -888,37 +886,23 @@ def analyze_var(
             else:
                 call_type = typ
 
+        # Bound variables with callable types are treated like methods
+        # (these are usually method aliases like __rmul__ = __mul__).
         if isinstance(call_type, FunctionLike) and not call_type.is_type_obj():
-            if mx.is_lvalue and not mx.suppress_errors:
-                if var.is_property and not var.is_settable_property:
-                    mx.msg.read_only_property(name, itype.type, mx.context)
-                elif not var.is_property:
-                    mx.msg.cant_assign_to_method(mx.context)
-
-            if not var.is_staticmethod:
-                # Class-level function objects and classmethods become bound methods:
-                # the former to the instance, the latter to the class.
-                functype: FunctionLike = call_type
-                signature = freshen_all_functions_type_vars(functype)
-                bound = get_proper_type(expand_self_type(var, signature, mx.original_type))
-                assert isinstance(bound, FunctionLike)
-                signature = bound
-                signature = check_self_arg(
-                    signature, mx.self_type, var.is_classmethod, mx.context, name, mx.msg
-                )
-                signature = bind_self(signature, mx.self_type, var.is_classmethod)
-                expanded_signature = expand_type_by_instance(signature, itype)
-                freeze_all_type_vars(expanded_signature)
-                if var.is_property:
-                    # A property cannot have an overloaded type => the cast is fine.
-                    assert isinstance(expanded_signature, CallableType)
-                    if var.is_settable_property and mx.is_lvalue and var.setter_type is not None:
-                        # TODO: use check_call() to infer better type, same as for __set__().
-                        result = expanded_signature.arg_types[0]
-                    else:
-                        result = expanded_signature.ret_type
+            if mx.is_lvalue and not var.is_property and not mx.suppress_errors:
+                mx.msg.cant_assign_to_method(mx.context)
+
+        # Bind the self type for each callable component (when needed).
+        if call_type and not var.is_staticmethod:
+            bound_items = []
+            for ct in call_type.items if isinstance(call_type, UnionType) else [call_type]:
+                p_ct = get_proper_type(ct)
+                if isinstance(p_ct, FunctionLike) and not p_ct.is_type_obj():
+                    item = expand_and_bind_callable(p_ct, var, itype, name, mx)
                 else:
-                    result = expanded_signature
+                    item = expand_without_binding(ct, var, itype, original_itype, mx)
+                bound_items.append(item)
+            result = UnionType.make_union(bound_items)
     else:
         if not var.is_ready and not mx.no_deferral:
             mx.not_ready_callback(var.name, mx.context)
@@ -937,6 +921,37 @@ def analyze_var(
     return result
 
 
+def expand_without_binding(
+    typ: Type, var: Var, itype: Instance, original_itype: Instance, mx: MemberContext
+) -> Type:
+    typ = freshen_all_functions_type_vars(typ)
+    typ = expand_self_type_if_needed(typ, mx, var, original_itype)
+    expanded = expand_type_by_instance(typ, itype)
+    freeze_all_type_vars(expanded)
+    return expanded
+
+
+def expand_and_bind_callable(
+    functype: FunctionLike, var: Var, itype: Instance, name: str, mx: MemberContext
+) -> Type:
+    functype = freshen_all_functions_type_vars(functype)
+    typ = get_proper_type(expand_self_type(var, functype, mx.original_type))
+    assert isinstance(typ, FunctionLike)
+    typ = check_self_arg(typ, mx.self_type, var.is_classmethod, mx.context, name, mx.msg)
+    typ = bind_self(typ, mx.self_type, var.is_classmethod)
+    expanded = expand_type_by_instance(typ, itype)
+    freeze_all_type_vars(expanded)
+    if not var.is_property:
+        return expanded
+    # TODO: a decorated property can result in Overloaded here.
+    assert isinstance(expanded, CallableType)
+    if var.is_settable_property and mx.is_lvalue and var.setter_type is not None:
+        # TODO: use check_call() to infer better type, same as for __set__().
+        return expanded.arg_types[0]
+    else:
+        return expanded.ret_type
+
+
 def expand_self_type_if_needed(
     t: Type, mx: MemberContext, var: Var, itype: Instance, is_class: bool = False
 ) -> Type:

test-data/unit/check-classvar.test

@@ -334,3 +334,12 @@ class C:
 c:C
 c.foo()  # E: Too few arguments \
          # N: "foo" is considered instance variable, to make it class variable use ClassVar[...]
+
+[case testClassVarUnionBoundOnInstance]
+from typing import Union, Callable, ClassVar
+
+class C:
+    def f(self) -> int: ...
+    g: ClassVar[Union[Callable[[C], int], int]] = f
+
+reveal_type(C().g)  # N: Revealed type is "Union[def () -> builtins.int, builtins.int]"