Infinite memory use on invalid program

[det]

struct Pair<A, B>
{
    a : A,
    b : B
}

fn pair<A, B>(a: A, b: B) -> Pair<A, B>
{
    Pair{ a : a, b: b }
}

trait Printable
{
    fn print(&self)
    {
        print!("{}", self)
    }
}


impl<A: Printable, B: Printable> Printable for Pair<A, B>
{
    fn print(&self)
    {
        self.a.print();
        self.b.print();
    }
}

fn print<A: Printable, B: Printable>(t: Pair<A, B>)
{
    t.print();
}

fn main()
{
    print(pair(1, 2));
}

[huonw]

I think this reduced example displays the problem more clearly: the compiler is trying to instantiate Pair<...> infinitely deeply. I guess the version with Pair<A, B> has exponential memory use due to doubling at each level.

struct Pair<A> {
    a: A,
}

trait Printable {
    fn print(&self) {}
}

impl<A: Printable> Printable for Pair<A> {}

// inlining/removing these stops the error
fn pair<A>(a: A) -> Pair<A> {
    Pair{ a : a }
}
fn print<A: Printable>(t: Pair<A>) {}

fn main() {
    print(pair(1));
}

<anon>:18:5: 18:10 error: overflow evaluating the trait `core::kinds::Sized` for the type `<generic #65>`
<anon>:18     print(pair(1));
              ^~~~~
<anon>:18:5: 18:10 note: the trait `core::kinds::Sized` must be implemented because it is required by `print`
<anon>:18     print(pair(1));
              ^~~~~
<anon>:18:5: 18:10 error: overflow evaluating the trait `Printable` for the type `<generic #65>`
<anon>:18     print(pair(1));
              ^~~~~
<anon>:18:5: 18:10 note: the trait `Printable` must be implemented because it is required by `print`
<anon>:18     print(pair(1));
              ^~~~~
<anon>:18:11: 18:18 error: mismatched types: expected `Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<<generic #65>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>`, found `Pair<<generic integer #0>>` (expected struct Pair, found integral variable)
<anon>:18     print(pair(1));
                    ^~~~~~~
error: aborting due to 3 previous errors
playpen: application terminated with error code 101

This happens even with a suffix on the 1, but does not happen if any other impl Printable is added, e.g. impl Printable for () {} will hit a proper (finite) type error, and impl Printable for uint {] will compile successfully. This makes me think it is caused by the new defaulting rules.

cc @nikomatsakis

[huonw]

This looks like it may be a dupe of #18400.

[arielb1]

This code properly gives an error on 0.12.

[nikomatsakis]

So, I don't see infinite memory use. I do see errors about overflow, which seems appropriate -- there is only one impl, and the compiler is trying to give the benefit of the doubt here.

[nikomatsakis]

I agree this is a dup of #18400