[red-knot] Understand typing.Tuple (#14927)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/red_knot_python_semantic/resources/mdtest/subscript/tuple.md

@@ -72,3 +72,55 @@ def _(m: int, n: int):
     # TODO: Support overloads... Should be `tuple[Literal[1, 'a', b"b"] | None, ...]`
     reveal_type(tuple_slice)  # revealed: @Todo(return type)
 ```
+
+## Inheritance
+
+```toml
+[environment]
+target-version = "3.9"
+```
+
+```py
+# TODO:
+#  * `tuple.__class_getitem__` is always bound on 3.9 (`sys.version_info`)
+#  * `tuple[int, str]` is a valid base (generics)
+# error: [call-possibly-unbound-method] "Method `__class_getitem__` of type `Literal[tuple]` is possibly unbound"
+# error: [invalid-base] "Invalid class base with type `GenericAlias` (all bases must be a class, `Any`, `Unknown` or `Todo`)"
+class A(tuple[int, str]): ...
+
+# Runtime value: `(A, tuple, object)`
+# TODO: Generics
+reveal_type(A.__mro__)  # revealed: tuple[Literal[A], Unknown, Literal[object]]
+```
+
+## `typing.Tuple`
+
+### Correspondence with `tuple`
+
+`typing.Tuple` can be used interchangeably with `tuple`:
+
+```py
+from typing import Tuple
+
+class A: ...
+
+def _(c: Tuple, d: Tuple[int, A], e: Tuple[Any, ...]):
+    reveal_type(c)  # revealed: tuple
+    reveal_type(d)  # revealed: tuple[int, A]
+    reveal_type(e)  # revealed: @Todo(full tuple[...] support)
+```
+
+### Inheritance
+
+Inheriting from `Tuple` results in a MRO with `builtins.tuple` and `typing.Generic`. `Tuple` itself
+is not a class.
+
+```py
+from typing import Tuple
+
+class C(Tuple): ...
+
+# Runtime value: `(C, tuple, typing.Generic, object)`
+# TODO: Add `Generic` to the MRO
+reveal_type(C.__mro__)  # revealed: tuple[Literal[C], Literal[tuple], Unknown, Literal[object]]
+```

crates/red_knot_python_semantic/src/types.rs

@@ -1758,6 +1758,7 @@ impl<'db> Type<'db> {
             Type::ClassLiteral(_) | Type::SubclassOf(_) => self.to_instance(db),
             // We treat `typing.Type` exactly the same as `builtins.type`:
             Type::KnownInstance(KnownInstanceType::Type) => KnownClass::Type.to_instance(db),
+            Type::KnownInstance(KnownInstanceType::Tuple) => KnownClass::Tuple.to_instance(db),
             Type::Union(union) => union.map(db, |element| element.in_type_expression(db)),
             Type::Unknown => Type::Unknown,
             // TODO map this to a new `Type::TypeVar` variant
@@ -2137,6 +2138,8 @@ pub enum KnownInstanceType<'db> {
     Never,
     /// The symbol `typing.Any` (which can also be found as `typing_extensions.Any`)
     Any,
+    /// The symbol `typing.Tuple` (which can also be found as `typing_extensions.Tuple`)
+    Tuple,
     /// The symbol `typing.Type` (which can also be found as `typing_extensions.Type`)
     Type,
     /// A single instance of `typing.TypeVar`
@@ -2157,6 +2160,7 @@ impl<'db> KnownInstanceType<'db> {
             Self::NoReturn => "NoReturn",
             Self::Never => "Never",
             Self::Any => "Any",
+            Self::Tuple => "Tuple",
             Self::Type => "Type",
             Self::TypeAliasType(_) => "TypeAliasType",
         }
@@ -2173,6 +2177,7 @@ impl<'db> KnownInstanceType<'db> {
             | Self::NoReturn
             | Self::Never
             | Self::Any
+            | Self::Tuple
             | Self::Type
             | Self::TypeAliasType(_) => Truthiness::AlwaysTrue,
         }
@@ -2188,6 +2193,7 @@ impl<'db> KnownInstanceType<'db> {
             Self::NoReturn => "typing.NoReturn",
             Self::Never => "typing.Never",
             Self::Any => "typing.Any",
+            Self::Tuple => "typing.Tuple",
             Self::Type => "typing.Type",
             Self::TypeVar(typevar) => typevar.name(db),
             Self::TypeAliasType(_) => "typing.TypeAliasType",
@@ -2204,7 +2210,8 @@ impl<'db> KnownInstanceType<'db> {
             Self::NoReturn => KnownClass::SpecialForm,
             Self::Never => KnownClass::SpecialForm,
             Self::Any => KnownClass::Object,
-            Self::Type => KnownClass::Object,
+            Self::Tuple => KnownClass::SpecialForm,
+            Self::Type => KnownClass::SpecialForm,
             Self::TypeVar(_) => KnownClass::TypeVar,
             Self::TypeAliasType(_) => KnownClass::TypeAliasType,
         }
@@ -2231,6 +2238,7 @@ impl<'db> KnownInstanceType<'db> {
             ("typing" | "typing_extensions", "Union") => Some(Self::Union),
             ("typing" | "typing_extensions", "NoReturn") => Some(Self::NoReturn),
             ("typing" | "typing_extensions", "Never") => Some(Self::Never),
+            ("typing" | "typing_extensions", "Tuple") => Some(Self::Tuple),
             ("typing" | "typing_extensions", "Type") => Some(Self::Type),
             _ => None,
         }

crates/red_knot_python_semantic/src/types/infer.rs

@@ -4862,6 +4862,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                 Type::Unknown
             }
             KnownInstanceType::Type => self.infer_subclass_of_type_expression(parameters),
+            KnownInstanceType::Tuple => self.infer_tuple_type_expression(parameters),
             KnownInstanceType::Any => Type::Any,
         }
     }

crates/red_knot_python_semantic/src/types/mro.rs

@@ -364,10 +364,13 @@ impl<'db> ClassBase<'db> {
                 | KnownInstanceType::Never
                 | KnownInstanceType::Optional => None,
                 KnownInstanceType::Any => Some(Self::Any),
-                // TODO: classes inheriting from `typing.Type` also have `Generic` in their MRO
+                // TODO: Classes inheriting from `typing.Type` et al. also have `Generic` in their MRO
                 KnownInstanceType::Type => {
                     ClassBase::try_from_ty(db, KnownClass::Type.to_class_literal(db))
                 }
+                KnownInstanceType::Tuple => {
+                    ClassBase::try_from_ty(db, KnownClass::Tuple.to_class_literal(db))
+                }
             },
         }
     }