From df1fc7ac38c8e73ceb5b95c613c83e4f55408d4f Mon Sep 17 00:00:00 2001
From: Louisvranderick <73151698+Louisvranderick@users.noreply.github.com>
Date: Fri, 10 Apr 2026 17:19:09 -0400
Subject: [PATCH] Fix NotImplemented incorrectly treated as callable (#2918)

NotImplementedType stubs inherit Any, so has_base_any made the singleton
callable via the implicit-Any call path. Exclude types.NotImplementedType
and builtins._NotImplementedType from that branch.

Tests: callable/calls regressions; return NotImplemented unchanged.
Made-with: Cursor
---
 pyrefly/lib/alt/call.rs      |  8 +++++++-
 pyrefly/lib/test/callable.rs | 10 ++++++++++
 pyrefly/lib/test/calls.rs    |  5 ++---
 3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/pyrefly/lib/alt/call.rs b/pyrefly/lib/alt/call.rs
index c89e8f0092..ca3ee322a2 100644
--- a/pyrefly/lib/alt/call.rs
+++ b/pyrefly/lib/alt/call.rs
@@ -366,9 +366,15 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
                     // If the class has an unknown base (e.g. inherits from an
                     // unresolved name), it might have inherited `__call__` from
                     // that base, so treat it as callable with implicit Any.
+                    //
+                    // `NotImplemented` is a singleton instance of `NotImplementedType`; it must
+                    // never be treated as callable even when stubs use `NotImplementedType(Any)`,
+                    // which would otherwise set `has_base_any` and hit this branch.
                     None if self
                         .get_metadata_for_class(cls.class_object())
-                        .has_base_any() =>
+                        .has_base_any()
+                        && !cls.has_qname("types", "NotImplementedType")
+                        && !cls.has_qname("builtins", "_NotImplementedType") =>
                     {
                         CallTargetLookup::Ok(Box::new(CallTarget::Any(AnyStyle::Implicit)))
                     }
diff --git a/pyrefly/lib/test/callable.rs b/pyrefly/lib/test/callable.rs
index 86dc0da691..8ed8716684 100644
--- a/pyrefly/lib/test/callable.rs
+++ b/pyrefly/lib/test/callable.rs
@@ -1449,3 +1449,13 @@ def after_func() -> None: ...
 schedule(1000, after_func)
 "#,
 );
+
+// Regression test for https://github.com/facebook/pyrefly/issues/2918
+testcase!(
+    test_notimplemented_not_callable,
+    r#"
+NotImplemented()  # E: Expected a callable
+NotImplemented("not yet done")  # E: Expected a callable
+raise NotImplementedError()
+"#,
+);
diff --git a/pyrefly/lib/test/calls.rs b/pyrefly/lib/test/calls.rs
index 9c5bee4b0e..20c8c3c402 100644
--- a/pyrefly/lib/test/calls.rs
+++ b/pyrefly/lib/test/calls.rs
@@ -408,16 +408,15 @@ def get_flow_version(run_id: str | None) -> str | None:
 
 // https://github.com/facebook/pyrefly/issues/2918
 testcase!(
-    bug = "Should error when calling NotImplemented (a constant, not a class)",
     test_call_not_implemented_constant,
     r#"
 # NotImplemented is a singleton constant, not a callable class.
 # Using NotImplemented() is always a mistake; they mean NotImplementedError().
 def broken():
-    raise NotImplemented()
+    raise NotImplemented()  # E: Expected a callable
 
 def also_broken():
-    raise NotImplemented("not yet done")
+    raise NotImplemented("not yet done")  # E: Expected a callable
 "#,
 );