[red-knot] Report invalid exceptions (astral-sh#15042)

Co-authored-by: Alex Waygood <[email protected]>

crates/red_knot_python_semantic/resources/mdtest/exception/basic.md

@@ -90,3 +90,83 @@ def foo(
         # TODO: should emit a diagnostic here:
         reveal_type(g)  # revealed: @Todo(full tuple[...] support)
 ```
+
+## Object raised is not an exception
+
+```py
+try:
+    raise AttributeError()  # fine
+except:
+    ...
+
+try:
+    raise FloatingPointError  # fine
+except:
+    ...
+
+try:
+    raise 1  # error: [invalid-raise]
+except:
+    ...
+
+try:
+    raise int  # error: [invalid-raise]
+except:
+    ...
+
+def _(e: Exception | type[Exception]):
+    raise e  # fine
+
+def _(e: Exception | type[Exception] | None):
+    raise e  # error: [invalid-raise]
+```
+
+## Exception cause is not an exception
+
+```py
+try:
+    raise EOFError() from GeneratorExit  # fine
+except:
+    ...
+
+try:
+    raise StopIteration from MemoryError()  # fine
+except:
+    ...
+
+try:
+    raise BufferError() from None  # fine
+except:
+    ...
+
+try:
+    raise ZeroDivisionError from False  # error: [invalid-raise]
+except:
+    ...
+
+try:
+    raise SystemExit from bool()  # error: [invalid-raise]
+except:
+    ...
+
+try:
+    raise
+except KeyboardInterrupt as e:  # fine
+    reveal_type(e)  # revealed: KeyboardInterrupt
+    raise LookupError from e  # fine
+
+try:
+    raise
+except int as e:  # error: [invalid-exception-caught]
+    reveal_type(e)  # revealed: Unknown
+    raise KeyError from e
+
+def _(e: Exception | type[Exception]):
+    raise ModuleNotFoundError from e  # fine
+
+def _(e: Exception | type[Exception] | None):
+    raise IndexError from e  # fine
+
+def _(e: int | None):
+    raise IndexError from e  # error: [invalid-raise]
+```

crates/red_knot_python_semantic/src/types.rs

@@ -2315,10 +2315,11 @@ impl<'db> KnownClass {
             .unwrap_or(Type::Unknown)
     }
 
-    pub fn to_subclass_of(self, db: &'db dyn Db) -> Option<Type<'db>> {
+    pub fn to_subclass_of(self, db: &'db dyn Db) -> Type<'db> {
         self.to_class_literal(db)
             .into_class_literal()
             .map(|ClassLiteralType { class }| Type::subclass_of(class))
+            .unwrap_or(Type::subclass_of_base(ClassBase::Unknown))
     }
 
     /// Return the module in which we should look up the definition for this class

crates/red_knot_python_semantic/src/types/diagnostic.rs

@@ -34,6 +34,7 @@ pub(crate) fn register_lints(registry: &mut LintRegistryBuilder) {
     registry.register_lint(&INVALID_DECLARATION);
     registry.register_lint(&INVALID_EXCEPTION_CAUGHT);
     registry.register_lint(&INVALID_PARAMETER_DEFAULT);
+    registry.register_lint(&INVALID_RAISE);
     registry.register_lint(&INVALID_TYPE_FORM);
     registry.register_lint(&INVALID_TYPE_VARIABLE_CONSTRAINTS);
     registry.register_lint(&NON_SUBSCRIPTABLE);
@@ -248,6 +249,49 @@ declare_lint! {
     }
 }
 
+declare_lint! {
+    /// Checks for `raise` statements that raise non-exceptions or use invalid
+    /// causes for their raised exceptions.
+    ///
+    /// ## Why is this bad?
+    /// Only subclasses or instances of `BaseException` can be raised.
+    /// For an exception's cause, the same rules apply, except that `None` is also
+    /// permitted. Violating these rules results in a `TypeError` at runtime.
+    ///
+    /// ## Examples
+    /// ```python
+    /// def f():
+    ///     try:
+    ///         something()
+    ///     except NameError:
+    ///         raise "oops!" from f
+    ///
+    /// def g():
+    ///     raise NotImplemented from 42
+    /// ```
+    ///
+    /// Use instead:
+    /// ```python
+    /// def f():
+    ///     try:
+    ///         something()
+    ///     except NameError as e:
+    ///         raise RuntimeError("oops!") from e
+    ///
+    /// def g():
+    ///     raise NotImplementedError from None
+    /// ```
+    ///
+    /// ## References
+    /// - [Python documentation: The `raise` statement](https://docs.python.org/3/reference/simple_stmts.html#raise)
+    /// - [Python documentation: Built-in Exceptions](https://docs.python.org/3/library/exceptions.html#built-in-exceptions)
+    pub(crate) static INVALID_RAISE = {
+        summary: "detects `raise` statements that raise invalid exceptions or use invalid causes",
+        status: LintStatus::preview("1.0.0"),
+        default_level: Level::Error,
+    }
+}
+
 declare_lint! {
     /// ## What it does
     /// Checks for invalid type expressions.
@@ -721,3 +765,26 @@ pub(super) fn report_invalid_exception_caught(context: &InferContext, node: &ast
         ),
     );
 }
+
+pub(crate) fn report_invalid_exception_raised(context: &InferContext, node: &ast::Expr, ty: Type) {
+    context.report_lint(
+        &INVALID_RAISE,
+        node.into(),
+        format_args!(
+            "Cannot raise object of type `{}` (must be a `BaseException` subclass or instance)",
+            ty.display(context.db())
+        ),
+    );
+}
+
+pub(crate) fn report_invalid_exception_cause(context: &InferContext, node: &ast::Expr, ty: Type) {
+    context.report_lint(
+        &INVALID_RAISE,
+        node.into(),
+        format_args!(
+            "Cannot use object of type `{}` as exception cause \
+            (must be a `BaseException` subclass or instance or `None`)",
+            ty.display(context.db())
+        ),
+    );
+}

crates/red_knot_python_semantic/src/types/infer.rs

@@ -74,7 +74,8 @@ use crate::Db;
 
 use super::context::{InferContext, WithDiagnostics};
 use super::diagnostic::{
-    report_index_out_of_bounds, report_invalid_exception_caught, report_non_subscriptable,
+    report_index_out_of_bounds, report_invalid_exception_caught, report_invalid_exception_cause,
+    report_invalid_exception_raised, report_non_subscriptable,
     report_possibly_unresolved_reference, report_slice_step_size_zero, report_unresolved_reference,
 };
 use super::string_annotation::{
@@ -1574,9 +1575,7 @@ impl<'db> TypeInferenceBuilder<'db> {
         // If it's an `except*` handler, this won't actually be the type of the bound symbol;
         // it will actually be the type of the generic parameters to `BaseExceptionGroup` or `ExceptionGroup`.
         let symbol_ty = if let Type::Tuple(tuple) = node_ty {
-            let type_base_exception = KnownClass::BaseException
-                .to_subclass_of(self.db())
-                .unwrap_or(Type::Unknown);
+            let type_base_exception = KnownClass::BaseException.to_subclass_of(self.db());
             let mut builder = UnionBuilder::new(self.db());
             for element in tuple.elements(self.db()).iter().copied() {
                 builder = builder.add(
@@ -1594,9 +1593,7 @@ impl<'db> TypeInferenceBuilder<'db> {
         } else if node_ty.is_subtype_of(self.db(), KnownClass::Tuple.to_instance(self.db())) {
             todo_type!("Homogeneous tuple in exception handler")
         } else {
-            let type_base_exception = KnownClass::BaseException
-                .to_subclass_of(self.db())
-                .unwrap_or(Type::Unknown);
+            let type_base_exception = KnownClass::BaseException.to_subclass_of(self.db());
             if node_ty.is_assignable_to(self.db(), type_base_exception) {
                 node_ty.to_instance(self.db())
             } else {
@@ -2198,8 +2195,30 @@ impl<'db> TypeInferenceBuilder<'db> {
             exc,
             cause,
         } = raise;
-        self.infer_optional_expression(exc.as_deref());
-        self.infer_optional_expression(cause.as_deref());
+
+        let base_exception_type = KnownClass::BaseException.to_subclass_of(self.db());
+        let base_exception_instance = base_exception_type.to_instance(self.db());
+
+        let can_be_raised =
+            UnionType::from_elements(self.db(), [base_exception_type, base_exception_instance]);
+        let can_be_exception_cause =
+            UnionType::from_elements(self.db(), [can_be_raised, Type::none(self.db())]);
+
+        if let Some(raised) = exc {
+            let raised_type = self.infer_expression(raised);
+
+            if !raised_type.is_assignable_to(self.db(), can_be_raised) {
+                report_invalid_exception_raised(&self.context, raised, raised_type);
+            }
+        }
+
+        if let Some(cause) = cause {
+            let cause_type = self.infer_expression(cause);
+
+            if !cause_type.is_assignable_to(self.db(), can_be_exception_cause) {
+                report_invalid_exception_cause(&self.context, cause, cause_type);
+            }
+        }
     }
 
     /// Given a `from .foo import bar` relative import, resolve the relative module