Support typing.NoReturn and typing.Never (#14559)

Fix #14558 
## Summary

- Add `typing.NoReturn` and `typing.Never` to known instances and infer
them as `Type::Never`
- Add `is_assignable_to` cases for `Type::Never`

I skipped emitting diagnostic for when a function is annotated as
`NoReturn` but it actually returns.

## Test Plan

Added tests from

https://github.com/python/typing/blob/main/conformance/tests/specialtypes_never.py
except from generics and checking if the return value of the function
and the annotations match.

---------

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

crates/red_knot_python_semantic/resources/mdtest/annotations/never.md

@@ -0,0 +1,62 @@
+# NoReturn & Never
+
+`NoReturn` is used to annotate the return type for functions that never return. `Never` is the
+bottom type, representing the empty set of Python objects. These two annotations can be used
+interchangeably.
+
+## Function Return Type Annotation
+
+```py
+from typing import NoReturn
+
+def stop() -> NoReturn:
+    raise RuntimeError("no way")
+
+# revealed: Never
+reveal_type(stop())
+```
+
+## Assignment
+
+```py
+from typing import NoReturn, Never, Any
+
+# error: [invalid-type-parameter] "Type `typing.Never` expected no type parameter"
+x: Never[int]
+a1: NoReturn
+# TODO: Test `Never` is only available in python >= 3.11
+a2: Never
+b1: Any
+b2: int
+
+def f():
+    # revealed: Never
+    reveal_type(a1)
+    # revealed: Never
+    reveal_type(a2)
+
+    # Never is assignable to all types.
+    v1: int = a1
+    v2: str = a1
+    # Other types are not assignable to Never except for Never (and Any).
+    v3: Never = b1
+    v4: Never = a2
+    v5: Any = b2
+    # error: [invalid-assignment] "Object of type `Literal[1]` is not assignable to `Never`"
+    v6: Never = 1
+```
+
+## Typing Extensions
+
+```py
+from typing_extensions import NoReturn, Never
+
+x: NoReturn
+y: Never
+
+def f():
+    # revealed: Never
+    reveal_type(x)
+    # revealed: Never
+    reveal_type(y)
+```

crates/red_knot_python_semantic/resources/mdtest/expression/if.md

@@ -27,7 +27,7 @@ reveal_type(1 if 0 else 2)  # revealed: Literal[2]
 
 (issue #14588)
 
-The test inside an if expression should not affect code outside of the block.
+The test inside an if expression should not affect code outside of the expression.
 
 ```py
 def bool_instance() -> bool:

crates/red_knot_python_semantic/src/types.rs

@@ -1553,6 +1553,9 @@ impl<'db> Type<'db> {
             // TODO map this to a new `Type::TypeVar` variant
             Type::KnownInstance(KnownInstanceType::TypeVar(_)) => *self,
             Type::KnownInstance(KnownInstanceType::TypeAliasType(alias)) => alias.value_ty(db),
+            Type::KnownInstance(KnownInstanceType::Never | KnownInstanceType::NoReturn) => {
+                Type::Never
+            }
             _ => todo_type!(),
         }
     }
@@ -1889,6 +1892,10 @@ pub enum KnownInstanceType<'db> {
     Optional,
     /// The symbol `typing.Union` (which can also be found as `typing_extensions.Union`)
     Union,
+    /// The symbol `typing.NoReturn` (which can also be found as `typing_extensions.NoReturn`)
+    NoReturn,
+    /// The symbol `typing.Never` available since 3.11 (which can also be found as `typing_extensions.Never`)
+    Never,
     /// A single instance of `typing.TypeVar`
     TypeVar(TypeVarInstance<'db>),
     /// A single instance of `typing.TypeAliasType` (PEP 695 type alias)
@@ -1899,11 +1906,13 @@ pub enum KnownInstanceType<'db> {
 impl<'db> KnownInstanceType<'db> {
     pub const fn as_str(self) -> &'static str {
         match self {
-            KnownInstanceType::Literal => "Literal",
-            KnownInstanceType::Optional => "Optional",
-            KnownInstanceType::Union => "Union",
-            KnownInstanceType::TypeVar(_) => "TypeVar",
-            KnownInstanceType::TypeAliasType(_) => "TypeAliasType",
+            Self::Literal => "Literal",
+            Self::Optional => "Optional",
+            Self::Union => "Union",
+            Self::TypeVar(_) => "TypeVar",
+            Self::NoReturn => "NoReturn",
+            Self::Never => "Never",
+            Self::TypeAliasType(_) => "TypeAliasType",
         }
     }
 
@@ -1914,6 +1923,8 @@ impl<'db> KnownInstanceType<'db> {
             | Self::Optional
             | Self::TypeVar(_)
             | Self::Union
+            | Self::NoReturn
+            | Self::Never
             | Self::TypeAliasType(_) => Truthiness::AlwaysTrue,
         }
     }
@@ -1924,6 +1935,8 @@ impl<'db> KnownInstanceType<'db> {
             Self::Literal => "typing.Literal",
             Self::Optional => "typing.Optional",
             Self::Union => "typing.Union",
+            Self::NoReturn => "typing.NoReturn",
+            Self::Never => "typing.Never",
             Self::TypeVar(typevar) => typevar.name(db),
             Self::TypeAliasType(_) => "typing.TypeAliasType",
         }
@@ -1935,6 +1948,8 @@ impl<'db> KnownInstanceType<'db> {
             Self::Literal => KnownClass::SpecialForm,
             Self::Optional => KnownClass::SpecialForm,
             Self::Union => KnownClass::SpecialForm,
+            Self::NoReturn => KnownClass::SpecialForm,
+            Self::Never => KnownClass::SpecialForm,
             Self::TypeVar(_) => KnownClass::TypeVar,
             Self::TypeAliasType(_) => KnownClass::TypeAliasType,
         }
@@ -1957,6 +1972,8 @@ impl<'db> KnownInstanceType<'db> {
             ("typing" | "typing_extensions", "Literal") => Some(Self::Literal),
             ("typing" | "typing_extensions", "Optional") => Some(Self::Optional),
             ("typing" | "typing_extensions", "Union") => Some(Self::Union),
+            ("typing" | "typing_extensions", "NoReturn") => Some(Self::NoReturn),
+            ("typing" | "typing_extensions", "Never") => Some(Self::Never),
             _ => None,
         }
     }

crates/red_knot_python_semantic/src/types/infer.rs

@@ -4575,7 +4575,7 @@ impl<'db> TypeInferenceBuilder<'db> {
 
         match value_ty {
             Type::KnownInstance(known_instance) => {
-                self.infer_parameterized_known_instance_type_expression(known_instance, slice)
+                self.infer_parameterized_known_instance_type_expression(subscript, known_instance)
             }
             _ => {
                 self.infer_type_expression(slice);
@@ -4586,9 +4586,10 @@ impl<'db> TypeInferenceBuilder<'db> {
 
     fn infer_parameterized_known_instance_type_expression(
         &mut self,
+        subscript: &ast::ExprSubscript,
         known_instance: KnownInstanceType,
-        parameters: &ast::Expr,
     ) -> Type<'db> {
+        let parameters = &*subscript.slice;
         match known_instance {
             KnownInstanceType::Literal => match self.infer_literal_parameter_type(parameters) {
                 Ok(ty) => ty,
@@ -4629,6 +4630,17 @@ impl<'db> TypeInferenceBuilder<'db> {
                 self.infer_type_expression(parameters);
                 todo_type!("generic type alias")
             }
+            KnownInstanceType::NoReturn | KnownInstanceType::Never => {
+                self.diagnostics.add(
+                    subscript.into(),
+                    "invalid-type-parameter",
+                    format_args!(
+                        "Type `{}` expected no type parameter",
+                        known_instance.repr(self.db)
+                    ),
+                );
+                Type::Unknown
+            }
         }
     }
 

crates/red_knot_python_semantic/src/types/mro.rs

@@ -375,6 +375,8 @@ impl<'db> ClassBase<'db> {
                 | KnownInstanceType::TypeAliasType(_)
                 | KnownInstanceType::Literal
                 | KnownInstanceType::Union
+                | KnownInstanceType::NoReturn
+                | KnownInstanceType::Never
                 | KnownInstanceType::Optional => None,
             },
         }