[red-knot] Improve type inference for iteration over heterogenous tuples (#13314)

Followup to #13295

crates/red_knot_python_semantic/src/types.rs

@@ -408,6 +408,18 @@ impl<'db> Type<'db> {
     ///     pass
     /// ```
     fn iterate(&self, db: &'db dyn Db) -> IterationOutcome<'db> {
+        if let Type::Tuple(tuple_type) = self {
+            return IterationOutcome::Iterable {
+                element_ty: tuple_type
+                    .elements(db)
+                    .iter()
+                    .fold(UnionBuilder::new(db), |builder, element| {
+                        builder.add(*element)
+                    })
+                    .build(),
+            };
+        }
+
         // `self` represents the type of the iterable;
         // `__iter__` and `__next__` are both looked up on the class of the iterable:
         let iterable_meta_type = self.to_meta_type(db);

crates/red_knot_python_semantic/src/types/infer.rs

@@ -4355,6 +4355,28 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn for_loop_with_heterogenous_tuple() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "src/a.py",
+            "
+            for x in (1, 'a', b'foo'):
+                pass
+            ",
+        )?;
+
+        assert_public_ty(
+            &db,
+            "src/a.py",
+            "x",
+            r#"Literal[1] | Literal["a"] | Literal[b"foo"]"#,
+        );
+
+        Ok(())
+    }
+
     #[test]
     fn except_handler_single_exception() -> anyhow::Result<()> {
         let mut db = setup_db();