[ty] Narrow specialized generics using isinstance() (#20256)

Closes astral-sh/ty#456. Part of astral-sh/ty#994.

After all the foundational work, this is only a small change, but let's
see if it exposes any unresolved issues.

crates/ty_python_semantic/resources/mdtest/exhaustiveness_checking.md

@@ -281,12 +281,10 @@ def if_else_exhaustive(x: A[D] | B[E] | C[F]):
     elif isinstance(x, C):
         pass
     else:
-        # TODO: both of these are false positives (https://github.com/astral-sh/ty/issues/456)
-        no_diagnostic_here  # error: [unresolved-reference]
-        assert_never(x)  # error: [type-assertion-failure]
+        no_diagnostic_here
+        assert_never(x)
 
-# TODO: false-positive diagnostic (https://github.com/astral-sh/ty/issues/456)
-def if_else_exhaustive_no_assertion(x: A[D] | B[E] | C[F]) -> int:  # error: [invalid-return-type]
+def if_else_exhaustive_no_assertion(x: A[D] | B[E] | C[F]) -> int:
     if isinstance(x, A):
         return 0
     elif isinstance(x, B):

crates/ty_python_semantic/resources/mdtest/narrow/isinstance.md

@@ -308,3 +308,88 @@ def i[T: Intersection[type[Bar], type[Baz | Spam]], U: (type[Eggs], type[Ham])](
 
     return (y, z)
 ```
+
+## Narrowing with generics
+
+```toml
+[environment]
+python-version = "3.12"
+```
+
+Narrowing to a generic class using `isinstance()` uses the top materialization of the generic. With
+a covariant generic, this is equivalent to using the upper bound of the type parameter (by default,
+`object`):
+
+```py
+class Covariant[T]:
+    def get(self) -> T:
+        raise NotImplementedError
+
+def _(x: object):
+    if isinstance(x, Covariant):
+        reveal_type(x)  # revealed: Covariant[object]
+        reveal_type(x.get())  # revealed: object
+```
+
+Similarly, contravariant type parameters use their lower bound of `Never`:
+
+```py
+class Contravariant[T]:
+    def push(self, x: T) -> None: ...
+
+def _(x: object):
+    if isinstance(x, Contravariant):
+        reveal_type(x)  # revealed: Contravariant[Never]
+        # error: [invalid-argument-type] "Argument to bound method `push` is incorrect: Expected `Never`, found `Literal[42]`"
+        x.push(42)
+```
+
+Invariant generics are trickiest. The top materialization, conceptually the type that includes all
+instances of the generic class regardless of the type parameter, cannot be represented directly in
+the type system, so we represent it with the internal `Top[]` special form.
+
+```py
+class Invariant[T]:
+    def push(self, x: T) -> None: ...
+    def get(self) -> T:
+        raise NotImplementedError
+
+def _(x: object):
+    if isinstance(x, Invariant):
+        reveal_type(x)  # revealed: Top[Invariant[Unknown]]
+        reveal_type(x.get())  # revealed: object
+        # error: [invalid-argument-type] "Argument to bound method `push` is incorrect: Expected `Never`, found `Literal[42]`"
+        x.push(42)
+```
+
+When more complex types are involved, the `Top[]` type may get simplified away.
+
+```py
+def _(x: list[int] | set[str]):
+    if isinstance(x, list):
+        reveal_type(x)  # revealed: list[int]
+    else:
+        reveal_type(x)  # revealed: set[str]
+```
+
+Though if the types involved are not disjoint bases, we necessarily keep a more complex type.
+
+```py
+def _(x: Invariant[int] | Covariant[str]):
+    if isinstance(x, Invariant):
+        reveal_type(x)  # revealed: Invariant[int] | (Covariant[str] & Top[Invariant[Unknown]])
+    else:
+        reveal_type(x)  # revealed: Covariant[str] & ~Top[Invariant[Unknown]]
+```
+
+The behavior of `issubclass()` is similar.
+
+```py
+def _(x: type[object], y: type[object], z: type[object]):
+    if issubclass(x, Covariant):
+        reveal_type(x)  # revealed: type[Covariant[object]]
+    if issubclass(y, Contravariant):
+        reveal_type(y)  # revealed: type[Contravariant[Never]]
+    if issubclass(z, Invariant):
+        reveal_type(z)  # revealed: type[Top[Invariant[Unknown]]]
+```

crates/ty_python_semantic/src/types/class.rs

@@ -29,10 +29,10 @@ use crate::types::typed_dict::typed_dict_params_from_class_def;
 use crate::types::{
     ApplyTypeMappingVisitor, Binding, BoundSuperError, BoundSuperType, CallableType,
     DataclassParams, DeprecatedInstance, FindLegacyTypeVarsVisitor, HasRelationToVisitor,
-    IsEquivalentVisitor, KnownInstanceType, ManualPEP695TypeAliasType, NormalizedVisitor,
-    PropertyInstanceType, StringLiteralType, TypeAliasType, TypeMapping, TypeRelation,
-    TypeVarBoundOrConstraints, TypeVarInstance, TypeVarKind, TypedDictParams, UnionBuilder,
-    VarianceInferable, declaration_type, infer_definition_types,
+    IsEquivalentVisitor, KnownInstanceType, ManualPEP695TypeAliasType, MaterializationKind,
+    NormalizedVisitor, PropertyInstanceType, StringLiteralType, TypeAliasType, TypeMapping,
+    TypeRelation, TypeVarBoundOrConstraints, TypeVarInstance, TypeVarKind, TypedDictParams,
+    UnionBuilder, VarianceInferable, declaration_type, infer_definition_types,
 };
 use crate::{
     Db, FxIndexMap, FxOrderSet, Program,
@@ -1470,6 +1470,18 @@ impl<'db> ClassLiteral<'db> {
         })
     }
 
+    pub(crate) fn top_materialization(self, db: &'db dyn Db) -> ClassType<'db> {
+        self.apply_specialization(db, |generic_context| {
+            generic_context
+                .default_specialization(db, self.known(db))
+                .materialize_impl(
+                    db,
+                    MaterializationKind::Top,
+                    &ApplyTypeMappingVisitor::default(),
+                )
+        })
+    }
+
     /// Returns the default specialization of this class. For non-generic classes, the class is
     /// returned unchanged. For a non-specialized generic class, we return a generic alias that
     /// applies the default specialization to the class's typevars.

crates/ty_python_semantic/src/types/narrow.rs

@@ -174,10 +174,10 @@ impl ClassInfoConstraintFunction {
     fn generate_constraint<'db>(self, db: &'db dyn Db, classinfo: Type<'db>) -> Option<Type<'db>> {
         let constraint_fn = |class: ClassLiteral<'db>| match self {
             ClassInfoConstraintFunction::IsInstance => {
-                Type::instance(db, class.default_specialization(db))
+                Type::instance(db, class.top_materialization(db))
             }
             ClassInfoConstraintFunction::IsSubclass => {
-                SubclassOfType::from(db, class.default_specialization(db))
+                SubclassOfType::from(db, class.top_materialization(db))
             }
         };