From f17ddd62ad191c2c164727ec3f30e485715d6cc6 Mon Sep 17 00:00:00 2001
From: Ibraheem Ahmed <ibraheem@ibraheem.ca>
Date: Fri, 24 Oct 2025 13:21:39 -0400
Subject: [PATCH] [ty] Avoid duplicate diagnostics during multi-inference of
 standalone expressions (#21056)

## Summary

Resolves https://github.com/astral-sh/ty/issues/1428.
---
 .../resources/mdtest/call/union.md              | 17 +++++++++++++++++
 crates/ty_python_semantic/src/types/context.rs  |  4 +++-
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/crates/ty_python_semantic/resources/mdtest/call/union.md b/crates/ty_python_semantic/resources/mdtest/call/union.md
index c950f7f482..1a4079204d 100644
--- a/crates/ty_python_semantic/resources/mdtest/call/union.md
+++ b/crates/ty_python_semantic/resources/mdtest/call/union.md
@@ -284,6 +284,8 @@ def _(flag: bool):
 
 Diagnostics unrelated to the type-context are only reported once:
 
+`expression.py`:
+
 ```py
 def f[T](x: T) -> list[T]:
     return [x]
@@ -307,3 +309,18 @@ def _(x: int):
     # error: [possibly-unresolved-reference] "Name `z` used when possibly not defined"
     y(f(True), [z])
 ```
+
+`standalone_expression.py`:
+
+```py
+def f(_: str): ...
+def g(_: str): ...
+def _(a: object, b: object, flag: bool):
+    if flag:
+        x = f
+    else:
+        x = g
+
+    # error: [unsupported-operator] "Operator `>` is not supported for types `object` and `object`"
+    x(f"{'a' if a > b else 'b'}")
+```
diff --git a/crates/ty_python_semantic/src/types/context.rs b/crates/ty_python_semantic/src/types/context.rs
index 2221ced32d..95e5ce8741 100644
--- a/crates/ty_python_semantic/src/types/context.rs
+++ b/crates/ty_python_semantic/src/types/context.rs
@@ -97,7 +97,9 @@ impl<'db, 'ast> InferContext<'db, 'ast> {
     }
 
     pub(crate) fn extend(&mut self, other: &TypeCheckDiagnostics) {
-        self.diagnostics.get_mut().extend(other);
+        if !self.is_in_multi_inference() {
+            self.diagnostics.get_mut().extend(other);
+        }
     }
 
     pub(super) fn is_lint_enabled(&self, lint: &'static LintMetadata) -> bool {