From e8ea40012afda872ef9bdf496a8b888ba80b533b Mon Sep 17 00:00:00 2001
From: Matthew Mckee <matthewmckee04@yahoo.co.uk>
Date: Tue, 3 Jun 2025 17:59:43 +0100
Subject: [PATCH] [ty] Add generic inference for dataclasses (#18443)

## Summary

An issue seen here https://github.com/astral-sh/ty/issues/500

The `__init__` method of dataclasses had no inherited generic context,
so we could not infer the type of an instance from a constructor call
with generics

## Test Plan

Add tests to classes.md` in generics folder
---
 .../resources/mdtest/generics/legacy/classes.md   | 15 +++++++++++++++
 .../resources/mdtest/generics/pep695/classes.md   | 12 ++++++++++++
 .../resources/mdtest/named_tuple.md               |  4 +---
 crates/ty_python_semantic/src/types/class.rs      |  3 ++-
 4 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/crates/ty_python_semantic/resources/mdtest/generics/legacy/classes.md b/crates/ty_python_semantic/resources/mdtest/generics/legacy/classes.md
index ad1fe65958..5e8858bf7b 100644
--- a/crates/ty_python_semantic/resources/mdtest/generics/legacy/classes.md
+++ b/crates/ty_python_semantic/resources/mdtest/generics/legacy/classes.md
@@ -379,6 +379,21 @@ C[None](b"bytes")  # error: [no-matching-overload]
 C[None](12)
 ```
 
+### Synthesized methods with dataclasses
+
+```py
+from dataclasses import dataclass
+from typing import Generic, TypeVar
+
+T = TypeVar("T")
+
+@dataclass
+class A(Generic[T]):
+    x: T
+
+reveal_type(A(x=1))  # revealed: A[int]
+```
+
 ## Generic subclass
 
 When a generic subclass fills its superclass's type parameter with one of its own, the actual types
diff --git a/crates/ty_python_semantic/resources/mdtest/generics/pep695/classes.md b/crates/ty_python_semantic/resources/mdtest/generics/pep695/classes.md
index 1d3fee229b..9726dbae54 100644
--- a/crates/ty_python_semantic/resources/mdtest/generics/pep695/classes.md
+++ b/crates/ty_python_semantic/resources/mdtest/generics/pep695/classes.md
@@ -354,6 +354,18 @@ C[None](b"bytes")  # error: [no-matching-overload]
 C[None](12)
 ```
 
+### Synthesized methods with dataclasses
+
+```py
+from dataclasses import dataclass
+
+@dataclass
+class A[T]:
+    x: T
+
+reveal_type(A(x=1))  # revealed: A[int]
+```
+
 ## Generic subclass
 
 When a generic subclass fills its superclass's type parameter with one of its own, the actual types
diff --git a/crates/ty_python_semantic/resources/mdtest/named_tuple.md b/crates/ty_python_semantic/resources/mdtest/named_tuple.md
index 8c5a79d62f..dfc81a73f0 100644
--- a/crates/ty_python_semantic/resources/mdtest/named_tuple.md
+++ b/crates/ty_python_semantic/resources/mdtest/named_tuple.md
@@ -134,9 +134,7 @@ class Property[T](NamedTuple):
     name: str
     value: T
 
-# TODO: this should be supported (no error, revealed type of `Property[float]`)
-# error: [invalid-argument-type]
-reveal_type(Property("height", 3.4))  # revealed: Property[Unknown]
+reveal_type(Property("height", 3.4))  # revealed: Property[float]
 ```
 
 ## Attributes on `NamedTuple`
diff --git a/crates/ty_python_semantic/src/types/class.rs b/crates/ty_python_semantic/src/types/class.rs
index bcf6c6e1b7..ab99ae27a6 100644
--- a/crates/ty_python_semantic/src/types/class.rs
+++ b/crates/ty_python_semantic/src/types/class.rs
@@ -1372,7 +1372,8 @@ impl<'db> ClassLiteral<'db> {
                 parameters.push(parameter);
             }
 
-            let signature = Signature::new(Parameters::new(parameters), Some(Type::none(db)));
+            let mut signature = Signature::new(Parameters::new(parameters), Some(Type::none(db)));
+            signature.inherited_generic_context = self.generic_context(db);
             Some(CallableType::function_like(db, signature))
         };