message: Add a test that fail/format works
  Summary: This was broken before the diffs below, so ensure it doesn't regress.
diff: @@ -123,3 +123,14 @@ res = [foo([])]
  assert!(errs.is_empty());
  assert_eq!(interface.get("res").unwrap(), &Ty::list(Ty::string()));
  }
  +
  +/// Test things that have previous claimed incorrectly they were type errors
  +#[test]
  +fn test_false_negative() {
  + let (errs, _, _, approx) = typecheck(
  + r#"fail("Expected variable expansion in string: `{}`".format("x"))"#,
  + &HashMap::new(),
  + );
  + assert!(approx.is_empty());
  + assert!(errs.is_empty());
  +}
  