Support __getitem__ type inference for subscripts (#13579)

## Summary

Follow-up to https://github.com/astral-sh/ruff/pull/13562, to add
support for "arbitrary" subscript operations.

crates/red_knot_python_semantic/src/types.rs

@@ -401,6 +401,16 @@ impl<'db> Type<'db> {
         }
     }
 
+    /// Return true if the type is a class or a union of classes.
+    pub fn is_class(&self, db: &'db dyn Db) -> bool {
+        match self {
+            Type::Union(union) => union.elements(db).iter().all(|ty| ty.is_class(db)),
+            Type::Class(_) => true,
+            // / TODO include type[X], once we add that type
+            _ => false,
+        }
+    }
+
     /// Return true if this type is a [subtype of] type `target`.
     ///
     /// [subtype of]: https://typing.readthedocs.io/en/latest/spec/concepts.html#subtype-supertype-and-type-equivalence

crates/red_knot_python_semantic/src/types/infer.rs

@@ -1322,6 +1322,22 @@ impl<'db> TypeInferenceBuilder<'db> {
         );
     }
 
+    /// Emit a diagnostic declaring that a type does not support subscripting.
+    pub(super) fn non_subscriptable_diagnostic(
+        &mut self,
+        node: AnyNodeRef,
+        non_subscriptable_ty: Type<'db>,
+    ) {
+        self.add_diagnostic(
+            node,
+            "non-subscriptable",
+            format_args!(
+                "Cannot subscript object of type '{}' with no `__getitem__` method.",
+                non_subscriptable_ty.display(self.db)
+            ),
+        );
+    }
+
     fn infer_for_statement_definition(
         &mut self,
         target: &ast::ExprName,
@@ -2588,7 +2604,35 @@ impl<'db> TypeInferenceBuilder<'db> {
                         Type::Unknown
                     })
             }
-            _ => Type::Todo,
+            (value_ty, slice_ty) => {
+                // Resolve the value to its class.
+                let value_meta_ty = value_ty.to_meta_type(self.db);
+
+                // If the class defines `__getitem__`, return its return type.
+                //
+                // See: https://docs.python.org/3/reference/datamodel.html#class-getitem-versus-getitem
+                let dunder_getitem_method = value_meta_ty.member(self.db, "__getitem__");
+                if !dunder_getitem_method.is_unbound() {
+                    return dunder_getitem_method
+                        .call(self.db, &[slice_ty])
+                        .unwrap_with_diagnostic(self.db, value.as_ref().into(), self);
+                }
+
+                // Otherwise, if the value is itself a class and defines `__class_getitem__`,
+                // return its return type.
+                if value_ty.is_class(self.db) {
+                    let dunder_class_getitem_method = value_ty.member(self.db, "__class_getitem__");
+                    if !dunder_class_getitem_method.is_unbound() {
+                        return dunder_class_getitem_method
+                            .call(self.db, &[slice_ty])
+                            .unwrap_with_diagnostic(self.db, value.as_ref().into(), self);
+                    }
+                }
+
+                // Otherwise, emit a diagnostic.
+                self.non_subscriptable_diagnostic((&**value).into(), value_ty);
+                Type::Unknown
+            }
         }
     }
 
@@ -6723,6 +6767,261 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn subscript_getitem_unbound() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "/src/a.py",
+            "
+                class NotSubscriptable:
+                    pass
+
+                a = NotSubscriptable()[0]
+            ",
+        )?;
+
+        assert_public_ty(&db, "/src/a.py", "a", "Unknown");
+        assert_file_diagnostics(
+            &db,
+            "/src/a.py",
+            &["Cannot subscript object of type 'NotSubscriptable' with no `__getitem__` method."],
+        );
+
+        Ok(())
+    }
+
+    #[test]
+    fn subscript_not_callable_getitem() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "/src/a.py",
+            "
+                class NotSubscriptable:
+                    __getitem__ = None
+
+                a = NotSubscriptable()[0]
+            ",
+        )?;
+
+        assert_public_ty(&db, "/src/a.py", "a", "Unknown");
+        assert_file_diagnostics(
+            &db,
+            "/src/a.py",
+            &["Object of type 'None' is not callable."],
+        );
+
+        Ok(())
+    }
+
+    #[test]
+    fn subscript_str_literal() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "/src/a.py",
+            "
+                def add(x: int, y: int) -> int:
+                    return x + y
+
+                a = 'abcde'[add(0, 1)]
+            ",
+        )?;
+
+        assert_public_ty(&db, "/src/a.py", "a", "str");
+
+        Ok(())
+    }
+
+    #[test]
+    fn subscript_getitem() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "/src/a.py",
+            "
+                class Identity:
+                    def __getitem__(self, index: int) -> int:
+                        return index
+
+                a = Identity()[0]
+            ",
+        )?;
+
+        assert_public_ty(&db, "/src/a.py", "a", "int");
+
+        Ok(())
+    }
+
+    #[test]
+    fn subscript_class_getitem() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "/src/a.py",
+            "
+                class Identity:
+                    def __class_getitem__(cls, item: int) -> str:
+                        return item
+
+                a = Identity[0]
+            ",
+        )?;
+
+        assert_public_ty(&db, "/src/a.py", "a", "str");
+
+        Ok(())
+    }
+
+    #[test]
+    fn subscript_getitem_union() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "/src/a.py",
+            "
+                flag = True
+
+                class Identity:
+                    if flag:
+                        def __getitem__(self, index: int) -> int:
+                            return index
+                    else:
+                        def __getitem__(self, index: int) -> str:
+                            return str(index)
+
+                a = Identity()[0]
+            ",
+        )?;
+
+        assert_public_ty(&db, "/src/a.py", "a", "int | str");
+
+        Ok(())
+    }
+
+    #[test]
+    fn subscript_class_getitem_union() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "/src/a.py",
+            "
+                flag = True
+
+                class Identity:
+                    if flag:
+                        def __class_getitem__(cls, item: int) -> str:
+                            return item
+                    else:
+                        def __class_getitem__(cls, item: int) -> int:
+                            return item
+
+                a = Identity[0]
+            ",
+        )?;
+
+        assert_public_ty(&db, "/src/a.py", "a", "str | int");
+
+        Ok(())
+    }
+
+    #[test]
+    fn subscript_class_getitem_class_union() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "/src/a.py",
+            "
+                flag = True
+
+                class Identity1:
+                    def __class_getitem__(cls, item: int) -> str:
+                        return item
+
+                class Identity2:
+                    def __class_getitem__(cls, item: int) -> int:
+                        return item
+
+                if flag:
+                    a = Identity1
+                else:
+                    a = Identity2
+
+                b = a[0]
+            ",
+        )?;
+
+        assert_public_ty(&db, "/src/a.py", "a", "Literal[Identity1, Identity2]");
+        assert_public_ty(&db, "/src/a.py", "b", "str | int");
+
+        Ok(())
+    }
+
+    #[test]
+    fn subscript_class_getitem_unbound_method_union() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "/src/a.py",
+            "
+                flag = True
+
+                if flag:
+                    class Identity:
+                        def __class_getitem__(self, x: int) -> str:
+                            pass
+                else:
+                    class Identity:
+                        pass
+
+                a = Identity[42]
+            ",
+        )?;
+
+        assert_public_ty(&db, "/src/a.py", "a", "str | Unknown");
+
+        assert_file_diagnostics(
+            &db,
+            "/src/a.py",
+            &["Object of type 'Literal[__class_getitem__] | Unbound' is not callable (due to union element 'Unbound')."],
+        );
+
+        Ok(())
+    }
+
+    #[test]
+    fn subscript_class_getitem_non_class_union() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "/src/a.py",
+            "
+                flag = True
+
+                if flag:
+                    class Identity:
+                        def __class_getitem__(self, x: int) -> str:
+                            pass
+                else:
+                    Identity = 1
+
+                a = Identity[42]
+            ",
+        )?;
+
+        // TODO this should _probably_ emit `str | Unknown` instead of `Unknown`.
+        assert_public_ty(&db, "/src/a.py", "a", "Unknown");
+
+        assert_file_diagnostics(
+            &db,
+            "/src/a.py",
+            &["Cannot subscript object of type 'Literal[Identity] | Literal[1]' with no `__getitem__` method."],
+        );
+
+        Ok(())
+    }
+
     #[test]
     fn dunder_call() -> anyhow::Result<()> {
         let mut db = setup_db();