From a8f7ccf2ca0959046ea53f44e35282920fdd58b8 Mon Sep 17 00:00:00 2001
From: Alex Waygood <Alex.Waygood@Gmail.com>
Date: Wed, 19 Nov 2025 19:27:12 +0000
Subject: [PATCH] [ty] Improve diagnostics when `NotImplemented` is called
 (#21523)

## Summary

Fixes https://github.com/astral-sh/ty/issues/1571.

I realised I was overcomplicating things when I described what we should
do in that issue description. The simplest thing to do here is just to
special-case call expressions and short-circuit the call-binding
machinery entirely if we see it's `NotImplemented` being called. It
doesn't really matter if the subdiagnostic doesn't fire when a union is
called and one element of the union is `NotImplemented` -- the
subdiagnostic doesn't need to be exhaustive; it's just to help people in
some common cases.

## Test Plan

Added snapshots
---
 .../resources/mdtest/call/builtins.md         |  5 +-
 ..._builtin_`NotImpl…_(ac366391ebdec9c0).snap | 47 +++++++++++++++++++
 .../src/types/infer/builder.rs                | 18 +++++++
 3 files changed, 69 insertions(+), 1 deletion(-)
 create mode 100644 crates/ty_python_semantic/resources/mdtest/snapshots/builtins.md_-_Calling_builtins_-_The_builtin_`NotImpl…_(ac366391ebdec9c0).snap

diff --git a/crates/ty_python_semantic/resources/mdtest/call/builtins.md b/crates/ty_python_semantic/resources/mdtest/call/builtins.md
index 742d275e3d..bac85f49d6 100644
--- a/crates/ty_python_semantic/resources/mdtest/call/builtins.md
+++ b/crates/ty_python_semantic/resources/mdtest/call/builtins.md
@@ -200,6 +200,9 @@ isinstance("", t.Any)  # error: [invalid-argument-type]
 
 ## The builtin `NotImplemented` constant is not callable
 
+<!-- snapshot-diagnostics -->
+
 ```py
-NotImplemented()  # error: [call-non-callable]
+raise NotImplemented()  # error: [call-non-callable]
+raise NotImplemented("this module is not implemented yet!!!")  # error: [call-non-callable]
 ```
diff --git a/crates/ty_python_semantic/resources/mdtest/snapshots/builtins.md_-_Calling_builtins_-_The_builtin_`NotImpl…_(ac366391ebdec9c0).snap b/crates/ty_python_semantic/resources/mdtest/snapshots/builtins.md_-_Calling_builtins_-_The_builtin_`NotImpl…_(ac366391ebdec9c0).snap
new file mode 100644
index 0000000000..0e6ea4d536
--- /dev/null
+++ b/crates/ty_python_semantic/resources/mdtest/snapshots/builtins.md_-_Calling_builtins_-_The_builtin_`NotImpl…_(ac366391ebdec9c0).snap
@@ -0,0 +1,47 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: builtins.md - Calling builtins - The builtin `NotImplemented` constant is not callable
+mdtest path: crates/ty_python_semantic/resources/mdtest/call/builtins.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | raise NotImplemented()  # error: [call-non-callable]
+2 | raise NotImplemented("this module is not implemented yet!!!")  # error: [call-non-callable]
+```
+
+# Diagnostics
+
+```
+error[call-non-callable]: `NotImplemented` is not callable
+ --> src/mdtest_snippet.py:1:7
+  |
+1 | raise NotImplemented()  # error: [call-non-callable]
+  |       --------------^^
+  |       |
+  |       Did you mean `NotImplementedError`?
+2 | raise NotImplemented("this module is not implemented yet!!!")  # error: [call-non-callable]
+  |
+info: rule `call-non-callable` is enabled by default
+
+```
+
+```
+error[call-non-callable]: `NotImplemented` is not callable
+ --> src/mdtest_snippet.py:2:7
+  |
+1 | raise NotImplemented()  # error: [call-non-callable]
+2 | raise NotImplemented("this module is not implemented yet!!!")  # error: [call-non-callable]
+  |       --------------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |       |
+  |       Did you mean `NotImplementedError`?
+  |
+info: rule `call-non-callable` is enabled by default
+
+```
diff --git a/crates/ty_python_semantic/src/types/infer/builder.rs b/crates/ty_python_semantic/src/types/infer/builder.rs
index 85c645d37a..3afbd10858 100644
--- a/crates/ty_python_semantic/src/types/infer/builder.rs
+++ b/crates/ty_python_semantic/src/types/infer/builder.rs
@@ -7912,6 +7912,24 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                 ty
             });
 
+        if callable_type.is_notimplemented(self.db()) {
+            if let Some(builder) = self
+                .context
+                .report_lint(&CALL_NON_CALLABLE, call_expression)
+            {
+                let mut diagnostic = builder.into_diagnostic("`NotImplemented` is not callable");
+                diagnostic.annotate(
+                    self.context
+                        .secondary(&**func)
+                        .message("Did you mean `NotImplementedError`?"),
+                );
+                diagnostic.set_concise_message(
+                    "`NotImplemented` is not callable - did you mean `NotImplementedError`?",
+                );
+            }
+            return Type::unknown();
+        }
+
         // Special handling for `TypedDict` method calls
         if let ast::Expr::Attribute(ast::ExprAttribute { value, attr, .. }) = func.as_ref() {
             let value_type = self.expression_type(value);