From b1ede8885bb55608231c5a188ce21aaa1eaf653b Mon Sep 17 00:00:00 2001
From: Douglas Creager <dcreager@dcreager.net>
Date: Tue, 9 Dec 2025 20:33:07 -0500
Subject: [PATCH] add more comments

---
 .../ty_python_semantic/src/types/generics.rs  | 26 +++++++++++++++++++
 .../src/types/signatures.rs                   |  4 +++
 2 files changed, 30 insertions(+)

diff --git a/crates/ty_python_semantic/src/types/generics.rs b/crates/ty_python_semantic/src/types/generics.rs
index 39ca666eab..5aaf2aad96 100644
--- a/crates/ty_python_semantic/src/types/generics.rs
+++ b/crates/ty_python_semantic/src/types/generics.rs
@@ -556,6 +556,13 @@ impl<'db> GenericContext<'db> {
                 let partial = PartialSpecialization {
                     generic_context: self,
                     types: &types,
+                    // Don't recursively substitute type[i] in itself. Ideally, we could instead
+                    // check if the result is self-referential after we're done applying the
+                    // partial specialization. But when we apply a paramspec, we don't use the
+                    // callable that it maps to directly; we create a new callable that reuses
+                    // parts of it. That means we can't look for the previous type directly.
+                    // Instead we use this to skip specializing the type in itself in the first
+                    // place.
                     skip: Some(i),
                 };
                 let updated = types[i].apply_type_mapping(
@@ -1362,6 +1369,8 @@ impl<'db> Specialization<'db> {
 pub struct PartialSpecialization<'a, 'db> {
     generic_context: GenericContext<'db>,
     types: &'a [Type<'db>],
+    /// An optional typevar to _not_ substitute when applying the specialization. We use this to
+    /// avoid recursively substituting a type inside of itself.
     skip: Option<usize>,
 }
 
@@ -1471,6 +1480,14 @@ impl<'db> SpecializationBuilder<'db> {
         }
     }
 
+    /// Finds all of the valid specializations of a constraint set, and adds their type mappings to
+    /// the specialization that this builder is building up.
+    ///
+    /// TODO: This is a stopgap! Eventually, the builder will maintain a single constraint set for
+    /// the main specialization that we are building, and [`build`][Self::build] will build the
+    /// specialization directly from that constraint set. This method lets us migrate to that brave
+    /// new world incrementally, by using the new constraint set mechanism piecemeal for certain
+    /// type comparisons.
     fn add_type_mappings_from_constraint_set(
         &mut self,
         constraints: ConstraintSet<'db>,
@@ -1526,6 +1543,15 @@ impl<'db> SpecializationBuilder<'db> {
         polarity: TypeVarVariance,
         mut f: &mut dyn FnMut(TypeVarAssignment<'db>) -> Option<Type<'db>>,
     ) -> Result<(), SpecializationError<'db>> {
+        // TODO: Eventually, the builder will maintain a constraint set, instead of a hash-map of
+        // type mappings, to represent the specialization that we are building up. At that point,
+        // this method will just need to compare `actual ≤ formal`, using constraint set
+        // assignability, and AND the result into the constraint set we are building.
+        //
+        // To make progress on that migration, we use constraint set assignability whenever
+        // possible when adding any new heuristics here. See the `Callable` clause below for an
+        // example.
+
         if formal == actual {
             return Ok(());
         }
diff --git a/crates/ty_python_semantic/src/types/signatures.rs b/crates/ty_python_semantic/src/types/signatures.rs
index dd84a5c987..7baeae28dc 100644
--- a/crates/ty_python_semantic/src/types/signatures.rs
+++ b/crates/ty_python_semantic/src/types/signatures.rs
@@ -443,6 +443,10 @@ impl<'db> CallableSignature<'db> {
             let self_is_single_paramspec = Self::signatures_is_single_paramspec(self_signatures);
             let other_is_single_paramspec = Self::signatures_is_single_paramspec(other_signatures);
 
+            // If either callable is a ParamSpec, the constraint set should bind the ParamSpec to
+            // the other callable's signature. We also need to compare the return types — for
+            // instance, to verify in `Callable[P, int]` that the return type is assignable to
+            // `int`, or in `Callable[P, T]` to bind `T` to the return type of the other callable.
             match (self_is_single_paramspec, other_is_single_paramspec) {
                 (
                     Some((self_bound_typevar, self_return_type)),