From efe668dc0b622363b0ab33f64533f4a7d037d4d1 Mon Sep 17 00:00:00 2001
From: devteamaegis <devteam.aegis@gmail.com>
Date: Thu, 13 Aug 2026 16:00:16 -0700
Subject: [PATCH] Don't resolve implicit `__bool__` through `__getattr__`

Implicit dunder methods such as `__bool__` are looked up on the type and
bypass `__getattr__` at runtime. When evaluating truthiness, Pyrefly
resolved `__bool__` with the `__getattr__` fallback enabled, so a class
whose `__getattr__` returns a non-callable type (e.g. `torch.nn.Module`,
whose `__getattr__` is typed to return `Tensor | Module`) was treated as
defining a non-callable `__bool__` and wrongly reported as `not-callable`
in boolean contexts.

Disable the `__getattr__` fallback when looking up `__bool__` in `as_bool`.

Closes #4467
---
 pyrefly/lib/alt/expr.rs        |  8 ++++++--
 pyrefly/lib/test/attributes.rs | 16 ++++++++++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/pyrefly/lib/alt/expr.rs b/pyrefly/lib/alt/expr.rs
index be18b8eab4..76db512864 100644
--- a/pyrefly/lib/alt/expr.rs
+++ b/pyrefly/lib/alt/expr.rs
@@ -1974,9 +1974,13 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
             }
         }
         ty.as_bool().or_else(|| {
-            // If the object defines `__bool__`, we can check if it returns a statically known value
+            // If the object defines `__bool__`, we can check if it returns a statically known value.
+            // Implicit dunder lookups (like the `__bool__` used for truthiness) are resolved on the
+            // type and do not go through `__getattr__`, so we disable the `__getattr__` fallback here.
+            // Otherwise a class with a `__getattr__` returning a non-callable type would be treated as
+            // defining a non-callable `__bool__` and wrongly reported as `not-callable` (see #4467).
             if self
-                .type_of_magic_dunder_attr(ty, &dunder::BOOL, range, errors, None, "as_bool", true)?
+                .type_of_magic_dunder_attr(ty, &dunder::BOOL, range, errors, None, "as_bool", false)?
                 .is_never()
             {
                 return None;
diff --git a/pyrefly/lib/test/attributes.rs b/pyrefly/lib/test/attributes.rs
index 6c19c98ca9..8df0bf5e74 100644
--- a/pyrefly/lib/test/attributes.rs
+++ b/pyrefly/lib/test/attributes.rs
@@ -944,6 +944,22 @@ def test(foo: Foo) -> None:
     "#,
 );
 
+testcase!(
+    test_getattr_does_not_provide_dunder_bool,
+    r#"
+# Implicit dunders like `__bool__` are looked up on the type and are not
+# resolved through `__getattr__`, so a `__getattr__` returning a non-callable
+# type must not make an instance look like it has a non-callable `__bool__`.
+# Regression test for https://github.com/facebook/pyrefly/issues/4467
+class Tensor: ...
+class M:
+    def __getattr__(self, name: str) -> "Tensor | M": ...
+
+def f(x: M | None) -> M:
+    return x or M()
+    "#,
+);
+
 testcase!(
     test_object_setattr_wrong_signature,
     r#"