From 96dbe609eb7f71de355873f0df688c756b1ab2ce Mon Sep 17 00:00:00 2001
From: Charlie Marsh <charlie.r.marsh@gmail.com>
Date: Wed, 17 Dec 2025 11:29:55 -0500
Subject: [PATCH] Respect deferred values in keyword arguments et al for .pyi
 files

---
 .../resources/mdtest/annotations/deferred.md  | 90 +++++++++++++++++++
 .../src/types/infer/builder.rs                | 15 ++++
 2 files changed, 105 insertions(+)

diff --git a/crates/ty_python_semantic/resources/mdtest/annotations/deferred.md b/crates/ty_python_semantic/resources/mdtest/annotations/deferred.md
index 89b9324ea4..0a1f8ea912 100644
--- a/crates/ty_python_semantic/resources/mdtest/annotations/deferred.md
+++ b/crates/ty_python_semantic/resources/mdtest/annotations/deferred.md
@@ -205,3 +205,93 @@ class B:
 class A(B): ...
 class B: ...
 ```
+
+## Default argument values
+
+### Not deferred in regular files
+
+```py
+# error: [unresolved-reference]
+def f(mode: int = ParseMode.test):
+    pass
+
+class ParseMode:
+    test = 1
+```
+
+### Deferred in stub files
+
+Forward references in default argument values are allowed in stub files.
+
+```pyi
+def f(mode: int = ParseMode.test): ...
+
+class ParseMode:
+    test: int
+```
+
+### Undefined names are still errors in stub files
+
+```pyi
+# error: [unresolved-reference]
+def f(mode: int = NeverDefined.test): ...
+```
+
+## Class keyword arguments
+
+### Not deferred in regular files
+
+```py
+# error: [unresolved-reference]
+class Foo(metaclass=SomeMeta):
+    pass
+
+class SomeMeta(type):
+    pass
+```
+
+### Deferred in stub files
+
+Forward references in class keyword arguments are allowed in stub files.
+
+```pyi
+class Foo(metaclass=SomeMeta): ...
+
+class SomeMeta(type): ...
+```
+
+### Undefined names are still errors in stub files
+
+```pyi
+# error: [unresolved-reference]
+class Foo(metaclass=NeverDefined): ...
+```
+
+## Lambda default argument values
+
+### Not deferred in regular files
+
+```py
+# error: [unresolved-reference]
+f = lambda x=Foo(): x
+
+class Foo:
+    pass
+```
+
+### Deferred in stub files
+
+Forward references in lambda default argument values are allowed in stub files.
+
+```pyi
+f = lambda x=Foo(): x
+
+class Foo: ...
+```
+
+### Undefined names are still errors in stub files
+
+```pyi
+# error: [unresolved-reference]
+f = lambda x=NeverDefined(): x
+```
diff --git a/crates/ty_python_semantic/src/types/infer/builder.rs b/crates/ty_python_semantic/src/types/infer/builder.rs
index c93269ade2..ac6d092c08 100644
--- a/crates/ty_python_semantic/src/types/infer/builder.rs
+++ b/crates/ty_python_semantic/src/types/infer/builder.rs
@@ -2358,12 +2358,16 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             decorator_types_and_nodes.push((decorator_type, decorator));
         }
 
+        // In stub files, default values may reference names that are defined later in the file.
+        let in_stub = self.in_stub();
+        let previous_deferred_state = std::mem::replace(&mut self.deferred_state, in_stub.into());
         for default in parameters
             .iter_non_variadic_params()
             .filter_map(|param| param.default.as_deref())
         {
             self.infer_expression(default, TypeContext::default());
         }
+        self.deferred_state = previous_deferred_state;
 
         // If there are type params, parameters and returns are evaluated in that scope. Otherwise,
         // we always defer the inference of the parameters and returns. That ensures that we do not
@@ -2998,9 +3002,14 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
         // if there are type parameters, then the keywords and bases are within that scope
         // and we don't need to run inference here
         if type_params.is_none() {
+            // In stub files, keyword values may reference names that are defined later in the file.
+            let in_stub = self.in_stub();
+            let previous_deferred_state =
+                std::mem::replace(&mut self.deferred_state, in_stub.into());
             for keyword in class_node.keywords() {
                 self.infer_expression(&keyword.value, TypeContext::default());
             }
+            self.deferred_state = previous_deferred_state;
 
             // Inference of bases deferred in stubs, or if any are string literals.
             if self.in_stub() || class_node.bases().iter().any(contains_string_literal) {
@@ -8322,6 +8331,10 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             body: _,
         } = lambda_expression;
 
+        // In stub files, default values may reference names that are defined later in the file.
+        let in_stub = self.in_stub();
+        let previous_deferred_state = std::mem::replace(&mut self.deferred_state, in_stub.into());
+
         let parameters = if let Some(parameters) = parameters {
             let positional_only = parameters
                 .posonlyargs
@@ -8387,6 +8400,8 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             Parameters::empty()
         };
 
+        self.deferred_state = previous_deferred_state;
+
         // TODO: Useful inference of a lambda's return type will require a different approach,
         // which does the inference of the body expression based on arguments at each call site,
         // rather than eagerly computing a return type without knowing the argument types.