[ty] Update SpecializationBuilder hook to get both lower/upper bounds

crates/ty_python_semantic/resources/mdtest/assignment/annotations.md

@@ -608,8 +608,7 @@ e: list[Any] | None = [1]
 reveal_type(e)  # revealed: list[Any]
 
 f: list[Any] | None = f2(1)
-# TODO: Better constraint solver.
-reveal_type(f)  # revealed: list[int] | None
+reveal_type(f)  # revealed: list[Any] | None
 
 g: list[Any] | dict[Any, Any] = f3(1)
 # TODO: Better constraint solver.

crates/ty_python_semantic/resources/mdtest/promotion.md

@@ -238,7 +238,7 @@ x11: list[Literal[1] | Literal[2] | Literal[3]] = [1, 2, 3]
 reveal_type(x11)  # revealed: list[Literal[1, 2, 3]]
 
 x12: Y[Y[Literal[1]]] = [[1]]
-reveal_type(x12)  # revealed: list[Y[Literal[1]]]
+reveal_type(x12)  # revealed: list[list[Literal[1]]]
 
 x13: list[tuple[Literal[1], Literal[2], Literal[3]]] = [(1, 2, 3)]
 reveal_type(x13)  # revealed: list[tuple[Literal[1], Literal[2], Literal[3]]]

crates/ty_python_semantic/src/types.rs

@@ -4,7 +4,6 @@ use ruff_diagnostics::{Edit, Fix};
 use rustc_hash::FxHashMap;
 
 use std::borrow::Cow;
-use std::cell::RefCell;
 use std::time::Duration;
 
 use bitflags::bitflags;
@@ -50,7 +49,7 @@ use crate::types::bound_super::BoundSuperType;
 use crate::types::call::{Binding, Bindings, CallArguments, CallableBinding};
 pub(crate) use crate::types::callable::{CallableType, CallableTypes};
 pub(crate) use crate::types::class_base::ClassBase;
-use crate::types::constraints::ConstraintSetBuilder;
+use crate::types::constraints::{ConstraintSetBuilder, Solutions};
 use crate::types::context::{LintDiagnosticGuard, LintDiagnosticGuardBuilder};
 use crate::types::diagnostic::{INVALID_AWAIT, INVALID_TYPE_FORM};
 pub use crate::types::display::{DisplaySettings, TypeDetail, TypeDisplayDetails};
@@ -59,10 +58,10 @@ use crate::types::function::{
     DataclassTransformerFlags, DataclassTransformerParams, FunctionDecorators, FunctionSpans,
     FunctionType, KnownFunction,
 };
+pub(crate) use crate::types::generics::GenericContext;
 use crate::types::generics::{
     ApplySpecialization, InferableTypeVars, Specialization, bind_typevar,
 };
-pub(crate) use crate::types::generics::{GenericContext, SpecializationBuilder};
 use crate::types::infer::InferenceFlags;
 use crate::types::known_instance::{InternedConstraintSet, InternedType, UnionTypeInstance};
 pub use crate::types::method::{BoundMethodType, KnownBoundMethodType, WrapperDescriptorKind};
@@ -1903,17 +1902,44 @@ impl<'db> Type<'db> {
         let generic_context = specialization.generic_context(db);
 
         // Collect the type mappings used to narrow the type context.
-        let tcx_mappings = {
-            let mut builder =
-                SpecializationBuilder::new(db, generic_context.inferable_typevars(db));
-
-            if let Some(tcx) = tcx.annotation {
+        //
+        // We use a forward CSA check (`alias_instance ≤ tcx`) to infer what each typevar
+        // in the identity specialization maps to in the type context. For example, if
+        // `tcx = list[int]` and `alias_instance = list[T]`, the CSA produces `T = int`.
+        let tcx_mappings: FxHashMap<_, _> = tcx
+            .annotation
+            .and_then(|tcx| {
                 let alias_instance = Type::instance(db, class_literal.identity_specialization(db));
-                let _ = builder.infer_reverse(constraints, tcx, alias_instance);
-            }
-
-            builder.into_type_mappings()
-        };
+                let inferable = generic_context.inferable_typevars(db);
+                let set = alias_instance.when_constraint_set_assignable_to(
+                    db,
+                    tcx,
+                    constraints,
+                    inferable,
+                );
+                match set.solutions(db, constraints) {
+                    Solutions::Constrained(solutions) => {
+                        let mut mappings = FxHashMap::default();
+                        for solution in solutions.iter() {
+                            for binding in solution {
+                                mappings
+                                    .entry(binding.bound_typevar.identity(db))
+                                    .and_modify(|existing| {
+                                        *existing = UnionType::from_two_elements(
+                                            db,
+                                            *existing,
+                                            binding.solution,
+                                        );
+                                    })
+                                    .or_insert(binding.solution);
+                            }
+                        }
+                        Some(mappings)
+                    }
+                    _ => None,
+                }
+            })
+            .unwrap_or_default();
 
         for (type_var, ty) in generic_context.variables(db).zip(specialization.types(db)) {
             let variance = type_var.variance_with_polarity(db, polarity);
@@ -5363,12 +5389,6 @@ impl<'db> Type<'db> {
                     match type_mapping {
                         TypeMapping::EagerExpansion => unreachable!("handled above"),
 
-                        // For UniqueSpecialization, get raw value type, apply specialization, then apply mapping.
-                        TypeMapping::UniqueSpecialization { .. } => {
-                            let value_type = alias.raw_value_type(db);
-                            alias.apply_function_specialization(db, value_type).apply_type_mapping_impl(db, type_mapping, tcx, visitor)
-                        }
-
                         _ => {
                             let value_type = alias.raw_value_type(db).apply_type_mapping_impl(db, type_mapping, tcx, visitor);
                             alias.apply_function_specialization(db, value_type).apply_type_mapping_impl(db, type_mapping, tcx, visitor)
@@ -5393,7 +5413,6 @@ impl<'db> Type<'db> {
             Type::LiteralValue(_) => match type_mapping {
                 TypeMapping::ApplySpecialization(_) |
                 TypeMapping::ApplySpecializationWithMaterialization { .. } |
-                TypeMapping::UniqueSpecialization { .. } |
                 TypeMapping::BindLegacyTypevars(_) |
                 TypeMapping::BindSelf { .. } |
                 TypeMapping::ReplaceSelf { .. } |
@@ -5408,7 +5427,6 @@ impl<'db> Type<'db> {
             Type::Dynamic(_) => match type_mapping {
                 TypeMapping::ApplySpecialization(_) |
                 TypeMapping::ApplySpecializationWithMaterialization { .. } |
-                TypeMapping::UniqueSpecialization { .. } |
                 TypeMapping::BindLegacyTypevars(_) |
                 TypeMapping::BindSelf(..) |
                 TypeMapping::ReplaceSelf { .. } |
@@ -6181,11 +6199,6 @@ pub enum TypeMapping<'a, 'db> {
         specialization: ApplySpecialization<'a, 'db>,
         materialization_kind: MaterializationKind,
     },
-    /// Resets any specializations to contain unique synthetic type variables.
-    UniqueSpecialization {
-        // A list of synthetic type variables, and the types they replaced.
-        specialization: RefCell<Vec<(BoundTypeVarInstance<'db>, Type<'db>)>>,
-    },
     /// Replaces any literal types with their corresponding promoted type form (e.g. `Literal["string"]`
     /// to `str`, or `def _() -> int` to `Callable[[], int]`).
     Promote(PromotionMode),
@@ -6237,8 +6250,7 @@ impl<'db> TypeMapping<'_, 'db> {
                     }),
                 )
             }
-            TypeMapping::UniqueSpecialization { .. }
-            | TypeMapping::Promote(_)
+            TypeMapping::Promote(_)
             | TypeMapping::BindLegacyTypevars(_)
             | TypeMapping::Materialize(_)
             | TypeMapping::ReplaceParameterDefaults
@@ -6283,7 +6295,6 @@ impl<'db> TypeMapping<'_, 'db> {
             },
             TypeMapping::Promote(mode) => TypeMapping::Promote(mode.flip()),
             TypeMapping::ApplySpecialization(_)
-            | TypeMapping::UniqueSpecialization { .. }
             | TypeMapping::BindLegacyTypevars(_)
             | TypeMapping::BindSelf(..)
             | TypeMapping::ReplaceSelf { .. }