From 1918c616237d792c242e28aeb66d9e6836f3dfa0 Mon Sep 17 00:00:00 2001
From: Carl Meyer <carl@astral.sh>
Date: Fri, 18 Apr 2025 06:46:21 -0700
Subject: [PATCH] [red-knot] class bases are not affected by
 __future__.annotations (#17456)

## Summary

We were over-conflating the conditions for deferred name resolution.
`from __future__ import annotations` defers annotations, but not class
bases. In stub files, class bases are also deferred. Modeling this
correctly also reduces likelihood of cycles in Python files using `from
__future__ import annotations` (since deferred resolution is inherently
cycle-prone). The same cycles are still possible in `.pyi` files, but
much less likely, since typically there isn't anything in a `pyi` file
that would cause an early return from a scope, or otherwise cause
visibility constraints to persist to end of scope. Usually there is only
code at module global scope and class scope, which can't have `return`
statements, and `raise` or `assert` statements in a stub file would be
very strange. (Technically according to the spec we'd be within our
rights to just forbid a whole bunch of syntax outright in a stub file,
but I kinda like minimizing unnecessary differences between the handling
of Python files and stub files.)

## Test Plan

Added mdtests.
---
 .../resources/mdtest/annotations/deferred.md  | 21 +++++++++++++++++++
 .../src/types/infer.rs                        | 12 +++++------
 2 files changed, 26 insertions(+), 7 deletions(-)

diff --git a/crates/red_knot_python_semantic/resources/mdtest/annotations/deferred.md b/crates/red_knot_python_semantic/resources/mdtest/annotations/deferred.md
index 7ee6e84060..8db8d90409 100644
--- a/crates/red_knot_python_semantic/resources/mdtest/annotations/deferred.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/annotations/deferred.md
@@ -156,3 +156,24 @@ def _():
         def f(self) -> C:
             return self
 ```
+
+## Base class references
+
+### Not deferred by __future__.annotations
+
+```py
+from __future__ import annotations
+
+class A(B):  # error: [unresolved-reference]
+    pass
+
+class B:
+    pass
+```
+
+### Deferred in stub files
+
+```pyi
+class A(B): ...
+class B: ...
+```
diff --git a/crates/red_knot_python_semantic/src/types/infer.rs b/crates/red_knot_python_semantic/src/types/infer.rs
index 29a92c058d..c2d5467c4c 100644
--- a/crates/red_knot_python_semantic/src/types/infer.rs
+++ b/crates/red_knot_python_semantic/src/types/infer.rs
@@ -585,8 +585,8 @@ impl<'db> TypeInferenceBuilder<'db> {
 
     /// Are we currently inferring types in file with deferred types?
     /// This is true for stub files and files with `__future__.annotations`
-    fn are_all_types_deferred(&self) -> bool {
-        self.index.has_future_annotations() || self.file().is_stub(self.db().upcast())
+    fn defer_annotations(&self) -> bool {
+        self.index.has_future_annotations() || self.in_stub()
     }
 
     /// Are we currently inferring deferred types?
@@ -1467,7 +1467,7 @@ impl<'db> TypeInferenceBuilder<'db> {
         // If there are type params, parameters and returns are evaluated in that scope, that is, in
         // `infer_function_type_params`, rather than here.
         if type_params.is_none() {
-            if self.are_all_types_deferred() {
+            if self.defer_annotations() {
                 self.types.deferred.insert(definition);
             } else {
                 self.infer_optional_annotation_expression(
@@ -1791,9 +1791,7 @@ impl<'db> TypeInferenceBuilder<'db> {
             // TODO: Only defer the references that are actually string literals, instead of
             // deferring the entire class definition if a string literal occurs anywhere in the
             // base class list.
-            if self.are_all_types_deferred()
-                || class_node.bases().iter().any(contains_string_literal)
-            {
+            if self.in_stub() || class_node.bases().iter().any(contains_string_literal) {
                 self.types.deferred.insert(definition);
             } else {
                 for base in class_node.bases() {
@@ -2919,7 +2917,7 @@ impl<'db> TypeInferenceBuilder<'db> {
 
         let mut declared_ty = self.infer_annotation_expression(
             annotation,
-            DeferredExpressionState::from(self.are_all_types_deferred()),
+            DeferredExpressionState::from(self.defer_annotations()),
         );
 
         if target