From 056258c767f8cf42eade8c23860cc63e51ee977c Mon Sep 17 00:00:00 2001
From: Douglas Creager <dcreager@dcreager.net>
Date: Fri, 5 Dec 2025 15:37:44 -0500
Subject: [PATCH] cs assignability for paramspecs

---
 .../mdtest/generics/pep695/paramspec.md       |  4 +-
 .../src/types/signatures.rs                   | 66 +++++++++++++++++--
 2 files changed, 63 insertions(+), 7 deletions(-)

diff --git a/crates/ty_python_semantic/resources/mdtest/generics/pep695/paramspec.md b/crates/ty_python_semantic/resources/mdtest/generics/pep695/paramspec.md
index 6483428bb3..f3d63d1689 100644
--- a/crates/ty_python_semantic/resources/mdtest/generics/pep695/paramspec.md
+++ b/crates/ty_python_semantic/resources/mdtest/generics/pep695/paramspec.md
@@ -392,15 +392,13 @@ def f3(x: int, y: str) -> int:
     return 1
 
 # TODO: This should reveal `(x: int, y: str) -> bool` but there's a cycle: https://github.com/astral-sh/ty/issues/1729
-reveal_type(f3)  # revealed: ((x: int, y: str) -> bool) | ((x: Divergent, y: Divergent) -> bool)
+reveal_type(f3)  # revealed: ((x: int, y: str) -> bool) | ((...) -> bool)
 
 reveal_type(f3(1, "a"))  # revealed: bool
 reveal_type(f3(x=1, y="a"))  # revealed: bool
 reveal_type(f3(1, y="a"))  # revealed: bool
 reveal_type(f3(y="a", x=1))  # revealed: bool
 
-# TODO: There should only be one error but the type of `f3` is a union: https://github.com/astral-sh/ty/issues/1729
-# error: [missing-argument] "No argument provided for required parameter `y`"
 # error: [missing-argument] "No argument provided for required parameter `y`"
 f3(1)
 # error: [invalid-argument-type] "Argument is incorrect: Expected `int`, found `Literal["a"]`"
diff --git a/crates/ty_python_semantic/src/types/signatures.rs b/crates/ty_python_semantic/src/types/signatures.rs
index 180a26efd6..42818f3012 100644
--- a/crates/ty_python_semantic/src/types/signatures.rs
+++ b/crates/ty_python_semantic/src/types/signatures.rs
@@ -29,10 +29,10 @@ use crate::types::generics::{
 };
 use crate::types::infer::nearest_enclosing_class;
 use crate::types::{
-    ApplyTypeMappingVisitor, BindingContext, BoundTypeVarInstance, CallableTypeKind, ClassLiteral,
-    FindLegacyTypeVarsVisitor, HasRelationToVisitor, IsDisjointVisitor, IsEquivalentVisitor,
-    KnownClass, MaterializationKind, NormalizedVisitor, ParamSpecAttrKind, TypeContext,
-    TypeMapping, TypeRelation, VarianceInferable, todo_type,
+    ApplyTypeMappingVisitor, BindingContext, BoundTypeVarInstance, CallableType, CallableTypeKind,
+    ClassLiteral, FindLegacyTypeVarsVisitor, HasRelationToVisitor, IsDisjointVisitor,
+    IsEquivalentVisitor, KnownClass, MaterializationKind, NormalizedVisitor, ParamSpecAttrKind,
+    TypeContext, TypeMapping, TypeRelation, VarianceInferable, todo_type,
 };
 use crate::{Db, FxOrderSet};
 use ruff_python_ast::{self as ast, name::Name};
@@ -1145,6 +1145,57 @@ impl<'db> Signature<'db> {
             return result;
         }
 
+        // If either parameter list is a ParamSpec, then we can create a constraint set that holds
+        // when the ParamSpec is assignable to the other parameter list.
+        if relation.is_constraint_set_assignability() {
+            match (
+                self.parameters.as_paramspec(),
+                other.parameters.as_paramspec(),
+            ) {
+                (Some(self_bound_typevar), Some(other_bound_typevar)) => {
+                    return ConstraintSet::constrain_typevar(
+                        db,
+                        self_bound_typevar,
+                        Type::TypeVar(other_bound_typevar),
+                        Type::TypeVar(other_bound_typevar),
+                        relation,
+                    );
+                }
+
+                (Some(self_bound_typevar), None) => {
+                    let upper = Type::Callable(CallableType::new(
+                        db,
+                        CallableSignature::single(Signature::new(other.parameters.clone(), None)),
+                        CallableTypeKind::ParamSpecValue,
+                    ));
+                    return ConstraintSet::constrain_typevar(
+                        db,
+                        self_bound_typevar,
+                        Type::Never,
+                        upper,
+                        relation,
+                    );
+                }
+
+                (None, Some(other_bound_typevar)) => {
+                    let lower = Type::Callable(CallableType::new(
+                        db,
+                        CallableSignature::single(Signature::new(self.parameters.clone(), None)),
+                        CallableTypeKind::ParamSpecValue,
+                    ));
+                    return ConstraintSet::constrain_typevar(
+                        db,
+                        other_bound_typevar,
+                        lower,
+                        Type::object(),
+                        relation,
+                    );
+                }
+
+                (None, None) => {}
+            }
+        }
+
         let mut parameters = ParametersZip {
             current_self: None,
             current_other: None,
@@ -1562,6 +1613,13 @@ impl<'db> Parameters<'db> {
         matches!(self.kind, ParametersKind::Gradual)
     }
 
+    pub(crate) const fn as_paramspec(&self) -> Option<BoundTypeVarInstance<'db>> {
+        match self.kind {
+            ParametersKind::ParamSpec(bound_typevar) => Some(bound_typevar),
+            _ => None,
+        }
+    }
+
     /// Return todo parameters: (*args: Todo, **kwargs: Todo)
     pub(crate) fn todo() -> Self {
         Self {