[ty] Fix panic on invalid syntax where we would incorrectly consider overloads in another file as belonging to a function in the file being checked

crates/ty_python_semantic/resources/mdtest/overloads.md

@@ -747,3 +747,30 @@ class Sub2(Base):
     # error: [invalid-overload]
     def method(self, x: str) -> str: ...
 ```
+
+### Invalid syntax regression case
+
+We used to panic on this snippet (see <https://github.com/astral-sh/ty/issues/1867>), because
+"iterating over the overloads" for `g` would incorrectly list the overloads for `f`:
+
+<!-- blacken-docs:off -->
+
+`module.pyi`:
+
+```pyi
+from typing import overload
+
+@overload
+def f() -> int: ...  # error: [invalid-overload]
+```
+
+`main.py`:
+
+```py
+import module
+g = module.f
+@staticmethod
+def g  # error: [invalid-syntax]
+```
+
+<!-- blacken-docs:on -->

crates/ty_python_semantic/src/types/function.rs

@@ -386,6 +386,15 @@ impl<'db> OverloadLiteral<'db> {
             return None;
         }
 
+        // These can both happen in edge cases involving not-yet-complete function definitions
+        // (invalid syntax).
+        if previous_overload.name(db) != self.name(db) {
+            return None;
+        }
+        if previous_overload.body_scope(db).file(db) != self.file(db) {
+            return None;
+        }
+
         Some(previous_literal)
     }