[ruff] Do not report when Optional has no type arguments (RUF013)

crates/ruff_linter/resources/test/fixtures/ruff/RUF013_4.py

@@ -0,0 +1,21 @@
+# https://github.com/astral-sh/ruff/issues/13833
+
+from typing import Optional
+
+
+def no_default(arg: Optional): ...
+
+
+def has_default(arg: Optional = None): ...
+
+
+def multiple_1(arg1: Optional, arg2: Optional = None): ...
+
+
+def multiple_2(arg1: Optional, arg2: Optional = None, arg3: int = None): ...
+
+
+def return_type(arg: Optional = None) -> Optional: ...
+
+
+def has_type_argument(arg: Optional[int] = None): ...

crates/ruff_linter/src/rules/ruff/mod.rs

@@ -32,6 +32,7 @@ mod tests {
     #[test_case(Rule::ImplicitOptional, Path::new("RUF013_1.py"))]
     #[test_case(Rule::ImplicitOptional, Path::new("RUF013_2.py"))]
     #[test_case(Rule::ImplicitOptional, Path::new("RUF013_3.py"))]
+    #[test_case(Rule::ImplicitOptional, Path::new("RUF013_4.py"))]
     #[test_case(Rule::MutableClassDefault, Path::new("RUF012.py"))]
     #[test_case(Rule::MutableDataclassDefault, Path::new("RUF008.py"))]
     #[test_case(Rule::ZipInsteadOfPairwise, Path::new("RUF007.py"))]

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__RUF013_RUF013_4.py.snap

@@ -0,0 +1,20 @@
+---
+source: crates/ruff_linter/src/rules/ruff/mod.rs
+snapshot_kind: text
+---
+RUF013_4.py:15:61: RUF013 [*] PEP 484 prohibits implicit `Optional`
+   |
+15 | def multiple_2(arg1: Optional, arg2: Optional = None, arg3: int = None): ...
+   |                                                             ^^^ RUF013
+   |
+   = help: Convert to `T | None`
+
+ℹ Unsafe fix
+12 12 | def multiple_1(arg1: Optional, arg2: Optional = None): ...
+13 13 | 
+14 14 | 
+15    |-def multiple_2(arg1: Optional, arg2: Optional = None, arg3: int = None): ...
+   15 |+def multiple_2(arg1: Optional, arg2: Optional = None, arg3: int | None = None): ...
+16 16 | 
+17 17 | 
+18 18 | def return_type(arg: Optional = None) -> Optional: ...

crates/ruff_linter/src/rules/ruff/typing.rs

@@ -227,6 +227,17 @@ impl<'a> TypingTarget<'a> {
     }
 }
 
+/// Whether `annotation` is `typing.Optional` with no type arguments.
+///
+/// See [#13833](https://github.com/astral-sh/ruff/issues/13833).
+fn annotation_is_bare_optional(checker: &Checker, annotation: &Expr) -> bool {
+    let Some(qualified_name) = checker.semantic().resolve_qualified_name(annotation) else {
+        return false;
+    };
+
+    matches!(qualified_name.segments(), ["typing", "Optional"])
+}
+
 /// Check if the given annotation [`Expr`] explicitly allows `None`.
 ///
 /// This function will return `None` if the annotation explicitly allows `None`
@@ -252,10 +263,14 @@ pub(crate) fn type_hint_explicitly_allows_none<'a>(
         }
         Some(target) => {
             if target.contains_none(checker, minor_version) {
-                None
-            } else {
-                Some(annotation)
+                return None;
             }
+
+            if annotation_is_bare_optional(checker, annotation) {
+                return None;
+            }
+
+            Some(annotation)
         }
     }
 }