[red-knot] Add support for `assert_never` (#17287)

## Summary

We already have partial "support" for `assert_never`, because it is
annotated as
```pyi
def assert_never(arg: Never, /) -> Never: ...
```
in typeshed. So we already emit a `invalid-argument-type` diagnostic if
the argument type to `assert_never` is not assignable to `Never`.

That is not enough, however. Gradual types like `Any`, `Unknown`,
`@Todo(…)` or `Any & int` can be assignable to `Never`. Which means that
we didn't issue any diagnostic in those cases.

Also, it seems like `assert_never` deserves a dedicated diagnostic
message, not just a generic "invalid argument type" error.

## Test Plan

New Markdown tests.

crates/red_knot_python_semantic/resources/mdtest/directives/assert_never.md

@@ -0,0 +1,106 @@
+# `assert_never`
+
+## Basic functionality
+
+`assert_never` makes sure that the type of the argument is `Never`. If it is not, a
+`type-assertion-failure` diagnostic is emitted.
+
+```py
+from typing_extensions import assert_never, Never, Any
+from knot_extensions import Unknown
+
+def _(never: Never, any_: Any, unknown: Unknown, flag: bool):
+    assert_never(never)  # fine
+
+    assert_never(0)  # error: [type-assertion-failure]
+    assert_never("")  # error: [type-assertion-failure]
+    assert_never(None)  # error: [type-assertion-failure]
+    assert_never([])  # error: [type-assertion-failure]
+    assert_never({})  # error: [type-assertion-failure]
+    assert_never(())  # error: [type-assertion-failure]
+    assert_never(1 if flag else never)  # error: [type-assertion-failure]
+
+    assert_never(any_)  # error: [type-assertion-failure]
+    assert_never(unknown)  # error: [type-assertion-failure]
+```
+
+## Use case: Type narrowing and exhaustiveness checking
+
+`assert_never` can be used in combination with type narrowing as a way to make sure that all cases
+are handled in a series of `isinstance` checks or other narrowing patterns that are supported.
+
+```py
+from typing_extensions import assert_never, Literal
+
+class A: ...
+class B: ...
+class C: ...
+
+def if_else_isinstance_success(obj: A | B):
+    if isinstance(obj, A):
+        pass
+    elif isinstance(obj, B):
+        pass
+    elif isinstance(obj, C):
+        pass
+    else:
+        assert_never(obj)
+
+def if_else_isinstance_error(obj: A | B):
+    if isinstance(obj, A):
+        pass
+    # B is missing
+    elif isinstance(obj, C):
+        pass
+    else:
+        # error: [type-assertion-failure] "Expected type `Never`, got `B & ~A & ~C` instead"
+        assert_never(obj)
+
+def if_else_singletons_success(obj: Literal[1, "a"] | None):
+    if obj == 1:
+        pass
+    elif obj == "a":
+        pass
+    elif obj is None:
+        pass
+    else:
+        assert_never(obj)
+
+def if_else_singletons_error(obj: Literal[1, "a"] | None):
+    if obj == 1:
+        pass
+    elif obj is "A":  # "A" instead of "a"
+        pass
+    elif obj is None:
+        pass
+    else:
+        # error: [type-assertion-failure] "Expected type `Never`, got `Literal["a"]` instead"
+        assert_never(obj)
+
+def match_singletons_success(obj: Literal[1, "a"] | None):
+    match obj:
+        case 1:
+            pass
+        case "a":
+            pass
+        case None:
+            pass
+        case _ as obj:
+            # TODO: Ideally, we would not emit an error here
+            # error: [type-assertion-failure] "Expected type `Never`, got `@Todo"
+            assert_never(obj)
+
+def match_singletons_error(obj: Literal[1, "a"] | None):
+    match obj:
+        case 1:
+            pass
+        case "A":  # "A" instead of "a"
+            pass
+        case None:
+            pass
+        case _ as obj:
+            # TODO: We should emit an error here, but the message should
+            # show the type `Literal["a"]` instead of `@Todo(…)`.
+            # error: [type-assertion-failure] "Expected type `Never`, got `@Todo"
+            assert_never(obj)
+```

crates/red_knot_python_semantic/src/types.rs

@@ -3024,6 +3024,24 @@ impl<'db> Type<'db> {
                     Signatures::single(signature)
                 }
 
+                Some(KnownFunction::AssertNever) => {
+                    let signature = CallableSignature::single(
+                        self,
+                        Signature::new(
+                            Parameters::new([Parameter::positional_only(Some(Name::new_static(
+                                "arg",
+                            )))
+                            // We need to set the type to `Any` here (instead of `Never`),
+                            // in order for every `assert_never` call to pass the argument
+                            // check. If we set it to `Never`, we'll get invalid-argument-type
+                            // errors instead of `type-assertion-failure` errors.
+                            .with_annotated_type(Type::any())]),
+                            Some(Type::none(db)),
+                        ),
+                    );
+                    Signatures::single(signature)
+                }
+
                 Some(KnownFunction::Cast) => {
                     let signature = CallableSignature::single(
                         self,
@@ -4890,6 +4908,8 @@ pub enum KnownFunction {
 
     /// `typing(_extensions).assert_type`
     AssertType,
+    /// `typing(_extensions).assert_never`
+    AssertNever,
     /// `typing(_extensions).cast`
     Cast,
     /// `typing(_extensions).overload`
@@ -4947,6 +4967,7 @@ impl KnownFunction {
         match self {
             Self::IsInstance | Self::IsSubclass | Self::Len | Self::Repr => module.is_builtins(),
             Self::AssertType
+            | Self::AssertNever
             | Self::Cast
             | Self::Overload
             | Self::RevealType
@@ -6407,6 +6428,7 @@ pub(crate) mod tests {
                 | KnownFunction::Overload
                 | KnownFunction::RevealType
                 | KnownFunction::AssertType
+                | KnownFunction::AssertNever
                 | KnownFunction::NoTypeCheck => KnownModule::TypingExtensions,
 
                 KnownFunction::IsSingleton

crates/red_knot_python_semantic/src/types/diagnostic.rs

@@ -682,7 +682,7 @@ declare_lint! {
 
 declare_lint! {
     /// ## What it does
-    /// Checks for `assert_type()` calls where the actual type
+    /// Checks for `assert_type()` and `assert_never()` calls where the actual type
     /// is not the same as the asserted type.
     ///
     /// ## Why is this bad?

crates/red_knot_python_semantic/src/types/infer.rs

@@ -4022,6 +4022,20 @@ impl<'db> TypeInferenceBuilder<'db> {
                                 }
                             }
                         }
+                        KnownFunction::AssertNever => {
+                            if let [Some(actual_ty)] = overload.parameter_types() {
+                                if !actual_ty.is_equivalent_to(self.db(), Type::Never) {
+                                    self.context.report_lint(
+                                        &TYPE_ASSERTION_FAILURE,
+                                        call_expression,
+                                        format_args!(
+                                            "Expected type `Never`, got `{}` instead",
+                                            actual_ty.display(self.db()),
+                                        ),
+                                    );
+                                }
+                            }
+                        }
                         KnownFunction::StaticAssert => {
                             if let [Some(parameter_ty), message] = overload.parameter_types() {
                                 let truthiness = match parameter_ty.try_bool(self.db()) {

knot.schema.json

@@ -652,7 +652,7 @@
         },
         "type-assertion-failure": {
           "title": "detects failed type assertions",
-          "description": "## What it does\nChecks for `assert_type()` calls where the actual type\nis not the same as the asserted type.\n\n## Why is this bad?\n`assert_type()` allows confirming the inferred type of a certain value.\n\n## Example\n\n```python\ndef _(x: int):\n    assert_type(x, int)  # fine\n    assert_type(x, str)  # error: Actual type does not match asserted type\n```",
+          "description": "## What it does\nChecks for `assert_type()` and `assert_never()` calls where the actual type\nis not the same as the asserted type.\n\n## Why is this bad?\n`assert_type()` allows confirming the inferred type of a certain value.\n\n## Example\n\n```python\ndef _(x: int):\n    assert_type(x, int)  # fine\n    assert_type(x, str)  # error: Actual type does not match asserted type\n```",
           "default": "error",
           "oneOf": [
             {