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// run-pass
// Regression test for #31299. This was generating an overflow error
// because of eager normalization:
//
// proving `M: Sized` requires
// - proving `PtrBack<Vec<M>>: Sized` requires
// - normalizing `Vec<<Vec<M> as Front>::Back>>: Sized` requires
// - proving `Vec<M>: Front` requires
// - `M: Sized` <-- cycle!
//
// If we skip the normalization step, though, everything goes fine.
//
// This could be fixed by implementing lazy normalization everywhere.
//
// However, we want this to work before then. For that, when checking
// whether a type is Sized we only check that the tails are Sized. As
// PtrBack does not have a tail, we don't need to normalize anything
// and this compiles
trait Front {
type Back;
}
impl<T> Front for Vec<T> {
type Back = Vec<T>;
}
struct PtrBack<T: Front>(#[allow(unused_tuple_struct_fields)] Vec<T::Back>);
struct M(#[allow(unused_tuple_struct_fields)] PtrBack<Vec<M>>);
#[allow(unused_must_use)]
fn main() {
std::mem::size_of::<M>();
}
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