[red-knot] Precise inference for __class__ attributes on objects of all types (#14921)

Author: AlexWaygood

crates/red_knot_python_semantic/resources/mdtest/attributes.md

@@ -115,3 +115,43 @@ def _(flag: bool):
     # error: [unresolved-attribute] "Type `Literal[C1, C2]` has no attribute `x`"
     reveal_type(C.x)  # revealed: Unknown
 ```
+
+## Objects of all types have a `__class__` method
+
+```py
+import typing
+
+reveal_type(typing.__class__)  # revealed: Literal[ModuleType]
+
+a = 42
+reveal_type(a.__class__)  # revealed: Literal[int]
+
+b = "42"
+reveal_type(b.__class__)  # revealed: Literal[str]
+
+c = b"42"
+reveal_type(c.__class__)  # revealed: Literal[bytes]
+
+d = True
+reveal_type(d.__class__)  # revealed: Literal[bool]
+
+e = (42, 42)
+reveal_type(e.__class__)  # revealed: Literal[tuple]
+
+def f(a: int, b: typing.LiteralString, c: int | str, d: type[str]):
+    reveal_type(a.__class__)  # revealed: type[int]
+    reveal_type(b.__class__)  # revealed: Literal[str]
+    reveal_type(c.__class__)  # revealed: type[int] | type[str]
+
+    # `type[type]`, a.k.a., either the class `type` or some subclass of `type`.
+    # It would be incorrect to infer `Literal[type]` here,
+    # as `c` could be some subclass of `str` with a custom metaclass.
+    # All we know is that the metaclass must be a (non-strict) subclass of `type`.
+    reveal_type(d.__class__)  # revealed: type[type]
+
+reveal_type(f.__class__)  # revealed: Literal[FunctionType]
+
+class Foo: ...
+
+reveal_type(Foo.__class__)  # revealed: Literal[type]
+```

crates/red_knot_python_semantic/resources/mdtest/scopes/moduletype_attrs.md

@@ -59,7 +59,7 @@ reveal_type(typing.__init__)  # revealed: Literal[__init__]
 # These come from `builtins.object`, not `types.ModuleType`:
 reveal_type(typing.__eq__)  # revealed: Literal[__eq__]
 
-reveal_type(typing.__class__)  # revealed: Literal[type]
+reveal_type(typing.__class__)  # revealed: Literal[ModuleType]
 
 # TODO: needs support for attribute access on instances, properties and generics;
 # should be `dict[str, Any]`

crates/red_knot_python_semantic/src/types.rs

@@ -1272,6 +1272,10 @@ impl<'db> Type<'db> {
     /// as accessed from instances of the `Bar` class.
     #[must_use]
     pub(crate) fn member(&self, db: &'db dyn Db, name: &str) -> Symbol<'db> {
+        if name == "__class__" {
+            return self.to_meta_type(db).into();
+        }
+
         match self {
             Type::Any => Type::Any.into(),
             Type::Never => {
@@ -2697,10 +2701,6 @@ impl<'db> Class<'db> {
             return Type::tuple(db, &tuple_elements).into();
         }
 
-        if name == "__class__" {
-            return self.metaclass(db).into();
-        }
-
         for superclass in self.iter_mro(db) {
             match superclass {
                 // TODO we may instead want to record the fact that we encountered dynamic, and intersect it with