overflow evaluating the requirement of ...

[Andlon]

I ran into some problems when attempting to get a blanket implementation for a trait of mine given certain requirements. I managed to reduce my code to the minimal example below.

trait Constraint {
    type Strategy;
}

impl Constraint for () {
    type Strategy = ();
}

struct Foo<T>(T);

impl<A> Constraint for Foo<A>
where
    A: Constraint,
{
    type Strategy = Foo<Option<A::Strategy>>;
}

impl<T> Constraint for Option<T>
where
    T: Constraint,
{
    type Strategy = Option<T::Strategy>;
}

trait DataStructure {
    type Constraint: Constraint;
}

impl<C> DataStructure for C
where
    C: Constraint<Strategy = C>,
{
    type Constraint = ();
}

impl<A> DataStructure for Foo<A>
where
    A: DataStructure,
{
    type Constraint = A::Constraint;
}

fn main() {}

I don't know too much about Rust internals, and I'm not really sure if this should be allowed, but I had perhaps expected that no actual recursion could occur in this case. What I get is the following compiler error:

error[E0275]: overflow evaluating the requirement `<std::option::Option<_> as Constraint>::Strategy`
  |
  = help: consider adding a `#![recursion_limit="128"]` attribute to your crate

error: aborting due to previous error

Rust compiler information:

rustc 1.23.0 (766bd11c8 2018-01-01)
binary: rustc
commit-hash: 766bd11c8a3c019ca53febdcd77b2215379dd67d
commit-date: 2018-01-01
host: x86_64-unknown-linux-gnu
release: 1.23.0
LLVM version: 4.0

Here's a playground link which demonstrates the issue: https://play.rust-lang.org/?gist=6d0b7b03aec50245d24de119f57dcafa&version=stable

I'm sorry if this has reported before. I searched and found many similar problems, but I simply don't have enough knowledge about the Rust internals to determine if it's the same problem or not.

[sgrif]

This is a duplicate of/related to #34260