Avoid flagging starred expressions in UP007 (#7505)

## Summary

These can't be fixed, because fixing them would lead to invalid syntax.
So flagging them also feels misleading.

Closes https://github.com/astral-sh/ruff/issues/7452.

crates/ruff/resources/test/fixtures/pyupgrade/UP007.py

@@ -108,3 +108,9 @@ class ServiceRefOrValue:
 # Regression test for: https://github.com/astral-sh/ruff/issues/7201
 class ServiceRefOrValue:
     service_specification: Optional[str]is not True = None
+
+
+# Regression test for: https://github.com/astral-sh/ruff/issues/7452
+class Collection(Protocol[*_B0]):
+    def __iter__(self) -> Iterator[Union[*_B0]]:
+        ...

crates/ruff/src/rules/pyupgrade/snapshots/ruff__rules__pyupgrade__tests__UP007.py.snap

@@ -410,5 +410,8 @@ UP007.py:110:28: UP007 [*] Use `X | Y` for type annotations
 109 109 | class ServiceRefOrValue:
 110     |-    service_specification: Optional[str]is not True = None
     110 |+    service_specification: str | None is not True = None
+111 111 | 
+112 112 | 
+113 113 | # Regression test for: https://github.com/astral-sh/ruff/issues/7452
 
 

crates/ruff_python_semantic/src/analyze/typing.rs

@@ -143,7 +143,7 @@ pub fn to_pep604_operator(
     slice: &Expr,
     semantic: &SemanticModel,
 ) -> Option<Pep604Operator> {
-    /// Returns `true` if any argument in the slice is a quoted annotation).
+    /// Returns `true` if any argument in the slice is a quoted annotation.
     fn quoted_annotation(slice: &Expr) -> bool {
         match slice {
             Expr::Constant(ast::ExprConstant {
@@ -155,6 +155,15 @@ pub fn to_pep604_operator(
         }
     }
 
+    /// Returns `true` if any argument in the slice is a starred expression.
+    fn starred_annotation(slice: &Expr) -> bool {
+        match slice {
+            Expr::Starred(_) => true,
+            Expr::Tuple(ast::ExprTuple { elts, .. }) => elts.iter().any(starred_annotation),
+            _ => false,
+        }
+    }
+
     // If the slice is a forward reference (e.g., `Optional["Foo"]`), it can only be rewritten
     // if we're in a typing-only context.
     //
@@ -175,6 +184,14 @@ pub fn to_pep604_operator(
         }
     }
 
+    // If any of the elements are starred expressions, we can't rewrite the subscript:
+    // ```python
+    // def f(x: Union[*int, str]): ...
+    // ```
+    if starred_annotation(slice) {
+        return None;
+    }
+
     semantic
         .resolve_call_path(value)
         .as_ref()