[ty] impl VarianceInferable for KnownInstanceType (#20924)

## Summary

Derived from #20900

Implement `VarianceInferable` for `KnownInstanceType` (especially for
`KnownInstanceType::TypeAliasType`).

The variance of a type alias matches its value type. In normal usage,
type aliases are expanded to value types, so the variance of a type
alias can be obtained without implementing this. However, for example,
if we want to display the variance when hovering over a type alias, we
need to be able to obtain the variance of the type alias itself (cf.
#20900).

## Test Plan

I couldn't come up with a way to test this in mdtest, so I'm testing it
in a test submodule at the end of `types.rs`.
I also added a test to `mdtest/generics/pep695/variance.md`, but it
passes without the changes in this PR.

crates/ty_python_semantic/resources/mdtest/generics/pep695/variance.md

@@ -790,6 +790,65 @@ static_assert(not is_assignable_to(C[B], C[A]))
 static_assert(not is_assignable_to(C[A], C[B]))
 ```
 
+## Type aliases
+
+The variance of the type alias matches the variance of the value type (RHS type).
+
+```py
+from ty_extensions import static_assert, is_subtype_of
+from typing import Literal
+
+class Covariant[T]:
+    def get(self) -> T:
+        raise ValueError
+
+type CovariantLiteral1 = Covariant[Literal[1]]
+type CovariantInt = Covariant[int]
+type MyCovariant[T] = Covariant[T]
+
+static_assert(is_subtype_of(CovariantLiteral1, CovariantInt))
+static_assert(is_subtype_of(MyCovariant[Literal[1]], MyCovariant[int]))
+
+class Contravariant[T]:
+    def set(self, value: T):
+        pass
+
+type ContravariantLiteral1 = Contravariant[Literal[1]]
+type ContravariantInt = Contravariant[int]
+type MyContravariant[T] = Contravariant[T]
+
+static_assert(is_subtype_of(ContravariantInt, ContravariantLiteral1))
+static_assert(is_subtype_of(MyContravariant[int], MyContravariant[Literal[1]]))
+
+class Invariant[T]:
+    def get(self) -> T:
+        raise ValueError
+
+    def set(self, value: T):
+        pass
+
+type InvariantLiteral1 = Invariant[Literal[1]]
+type InvariantInt = Invariant[int]
+type MyInvariant[T] = Invariant[T]
+
+static_assert(not is_subtype_of(InvariantInt, InvariantLiteral1))
+static_assert(not is_subtype_of(InvariantLiteral1, InvariantInt))
+static_assert(not is_subtype_of(MyInvariant[Literal[1]], MyInvariant[int]))
+static_assert(not is_subtype_of(MyInvariant[int], MyInvariant[Literal[1]]))
+
+class Bivariant[T]:
+    pass
+
+type BivariantLiteral1 = Bivariant[Literal[1]]
+type BivariantInt = Bivariant[int]
+type MyBivariant[T] = Bivariant[T]
+
+static_assert(is_subtype_of(BivariantInt, BivariantLiteral1))
+static_assert(is_subtype_of(BivariantLiteral1, BivariantInt))
+static_assert(is_subtype_of(MyBivariant[Literal[1]], MyBivariant[int]))
+static_assert(is_subtype_of(MyBivariant[int], MyBivariant[Literal[1]]))
+```
+
 ## Inheriting from generic classes with inferred variance
 
 When inheriting from a generic class with our type variable substituted in, we count its occurrences

crates/ty_python_semantic/src/types.rs

@@ -55,9 +55,10 @@ use crate::types::function::{
     DataclassTransformerFlags, DataclassTransformerParams, FunctionSpans, FunctionType,
     KnownFunction,
 };
+pub(crate) use crate::types::generics::GenericContext;
 use crate::types::generics::{
-    GenericContext, InferableTypeVars, PartialSpecialization, Specialization, bind_typevar,
-    typing_self, walk_generic_context,
+    InferableTypeVars, PartialSpecialization, Specialization, bind_typevar, typing_self,
+    walk_generic_context,
 };
 use crate::types::infer::infer_unpack_types;
 use crate::types::mro::{Mro, MroError, MroIterator};
@@ -7274,6 +7275,7 @@ impl<'db> VarianceInferable<'db> for Type<'db> {
                 .collect(),
             Type::SubclassOf(subclass_of_type) => subclass_of_type.variance_of(db, typevar),
             Type::TypeIs(type_is_type) => type_is_type.variance_of(db, typevar),
+            Type::KnownInstance(known_instance) => known_instance.variance_of(db, typevar),
             Type::Dynamic(_)
             | Type::Never
             | Type::WrapperDescriptor(_)
@@ -7288,7 +7290,6 @@ impl<'db> VarianceInferable<'db> for Type<'db> {
             | Type::LiteralString
             | Type::BytesLiteral(_)
             | Type::SpecialForm(_)
-            | Type::KnownInstance(_)
             | Type::AlwaysFalsy
             | Type::AlwaysTruthy
             | Type::BoundSuper(_)
@@ -7495,6 +7496,17 @@ fn walk_known_instance_type<'db, V: visitor::TypeVisitor<'db> + ?Sized>(
     }
 }
 
+impl<'db> VarianceInferable<'db> for KnownInstanceType<'db> {
+    fn variance_of(self, db: &'db dyn Db, typevar: BoundTypeVarInstance<'db>) -> TypeVarVariance {
+        match self {
+            KnownInstanceType::TypeAliasType(type_alias) => {
+                type_alias.raw_value_type(db).variance_of(db, typevar)
+            }
+            _ => TypeVarVariance::Bivariant,
+        }
+    }
+}
+
 impl<'db> KnownInstanceType<'db> {
     fn normalized_impl(self, db: &'db dyn Db, visitor: &NormalizedVisitor<'db>) -> Self {
         match self {
@@ -10693,14 +10705,10 @@ impl<'db> PEP695TypeAliasType<'db> {
         semantic_index(db, scope.file(db)).expect_single_definition(type_alias_stmt_node)
     }
 
+    /// The RHS type of a PEP-695 style type alias with specialization applied.
     #[salsa::tracked(cycle_fn=value_type_cycle_recover, cycle_initial=value_type_cycle_initial, heap_size=ruff_memory_usage::heap_size)]
     pub(crate) fn value_type(self, db: &'db dyn Db) -> Type<'db> {
-        let scope = self.rhs_scope(db);
-        let module = parsed_module(db, scope.file(db)).load(db);
-        let type_alias_stmt_node = scope.node(db).expect_type_alias();
-        let definition = self.definition(db);
-        let value_type =
-            definition_expression_type(db, definition, &type_alias_stmt_node.node(&module).value);
+        let value_type = self.raw_value_type(db);
 
         if let Some(generic_context) = self.generic_context(db) {
             let specialization = self
@@ -10713,6 +10721,25 @@ impl<'db> PEP695TypeAliasType<'db> {
         }
     }
 
+    /// The RHS type of a PEP-695 style type alias with *no* specialization applied.
+    ///
+    /// ## Warning
+    ///
+    /// This uses the semantic index to find the definition of the type alias. This means that if the
+    /// calling query is not in the same file as this type alias is defined in, then this will create
+    /// a cross-module dependency directly on the full AST which will lead to cache
+    /// over-invalidation.
+    /// This method also calls the type inference functions, and since type aliases can have recursive structures,
+    /// we should be careful not to create infinite recursions in this method (or make it tracked if necessary).
+    pub(crate) fn raw_value_type(self, db: &'db dyn Db) -> Type<'db> {
+        let scope = self.rhs_scope(db);
+        let module = parsed_module(db, scope.file(db)).load(db);
+        let type_alias_stmt_node = scope.node(db).expect_type_alias();
+        let definition = self.definition(db);
+
+        definition_expression_type(db, definition, &type_alias_stmt_node.node(&module).value)
+    }
+
     pub(crate) fn apply_specialization(
         self,
         db: &'db dyn Db,
@@ -10892,6 +10919,13 @@ impl<'db> TypeAliasType<'db> {
         }
     }
 
+    pub(crate) fn raw_value_type(self, db: &'db dyn Db) -> Type<'db> {
+        match self {
+            TypeAliasType::PEP695(type_alias) => type_alias.raw_value_type(db),
+            TypeAliasType::ManualPEP695(type_alias) => type_alias.value(db),
+        }
+    }
+
     pub(crate) fn as_pep_695_type_alias(self) -> Option<PEP695TypeAliasType<'db>> {
         match self {
             TypeAliasType::PEP695(type_alias) => Some(type_alias),
@@ -11724,4 +11758,85 @@ pub(crate) mod tests {
             .build();
         assert_eq!(intersection.display(&db).to_string(), "Never");
     }
+
+    #[test]
+    fn type_alias_variance() {
+        use crate::db::tests::TestDb;
+        use crate::place::global_symbol;
+
+        fn get_type_alias<'db>(db: &'db TestDb, name: &str) -> PEP695TypeAliasType<'db> {
+            let module = ruff_db::files::system_path_to_file(db, "/src/a.py").unwrap();
+            let ty = global_symbol(db, module, name).place.expect_type();
+            let Type::KnownInstance(KnownInstanceType::TypeAliasType(TypeAliasType::PEP695(
+                type_alias,
+            ))) = ty
+            else {
+                panic!("Expected `{name}` to be a type alias");
+            };
+            type_alias
+        }
+        fn get_bound_typevar<'db>(
+            db: &'db TestDb,
+            type_alias: PEP695TypeAliasType<'db>,
+        ) -> BoundTypeVarInstance<'db> {
+            let generic_context = type_alias.generic_context(db).unwrap();
+            generic_context.variables(db).next().unwrap()
+        }
+
+        let mut db = setup_db();
+        db.write_dedented(
+            "/src/a.py",
+            r#"
+class Covariant[T]:
+    def get(self) -> T:
+        raise ValueError
+
+class Contravariant[T]:
+    def set(self, value: T):
+        pass
+
+class Invariant[T]:
+    def get(self) -> T:
+        raise ValueError
+    def set(self, value: T):
+        pass
+
+class Bivariant[T]:
+    pass
+
+type CovariantAlias[T] = Covariant[T]
+type ContravariantAlias[T] = Contravariant[T]
+type InvariantAlias[T] = Invariant[T]
+type BivariantAlias[T] = Bivariant[T]
+"#,
+        )
+        .unwrap();
+        let covariant = get_type_alias(&db, "CovariantAlias");
+        assert_eq!(
+            KnownInstanceType::TypeAliasType(TypeAliasType::PEP695(covariant))
+                .variance_of(&db, get_bound_typevar(&db, covariant)),
+            TypeVarVariance::Covariant
+        );
+
+        let contravariant = get_type_alias(&db, "ContravariantAlias");
+        assert_eq!(
+            KnownInstanceType::TypeAliasType(TypeAliasType::PEP695(contravariant))
+                .variance_of(&db, get_bound_typevar(&db, contravariant)),
+            TypeVarVariance::Contravariant
+        );
+
+        let invariant = get_type_alias(&db, "InvariantAlias");
+        assert_eq!(
+            KnownInstanceType::TypeAliasType(TypeAliasType::PEP695(invariant))
+                .variance_of(&db, get_bound_typevar(&db, invariant)),
+            TypeVarVariance::Invariant
+        );
+
+        let bivariant = get_type_alias(&db, "BivariantAlias");
+        assert_eq!(
+            KnownInstanceType::TypeAliasType(TypeAliasType::PEP695(bivariant))
+                .variance_of(&db, get_bound_typevar(&db, bivariant)),
+            TypeVarVariance::Bivariant
+        );
+    }
 }