Function could_unify_impl 

fn could_unify_impl<'db>(
    db: &'db dyn HirDatabase,
    env: ParamEnvAndCrate<'db>,
    tys: &Canonical<'db, (Ty<'db>, Ty<'db>)>,
    select: for<'a> fn(&mut ObligationCtxt<'a, 'db>) -> Vec<NextSolverError<'db>>,
) -> bool