Compiler unable to determine two types are equal

[grantslatton]

The compiler seems unable to determine that two types are equal.

I tried this code:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=b363f89f0e686e243f3a92dcc1cfc09a

trait Factory {
    type Product;
}

impl Factory for () {
    type Product = ();
}

trait ProductConsumer<P> {
    fn consume(self, product: P);
}

impl <P> ProductConsumer<P> for () {
    fn consume(self, product: P) {}
}


fn make_product_consumer<F: Factory>(f: F) -> impl ProductConsumer<F::Product> {
    ()
}

fn main() {
    let consumer = make_product_consumer(());
    consumer.consume(());
}

I expected the program to compile and run without issue, because <() as Factory>::Product> is (), so the consume call should be fine.

Instead, I got this error message:

error[E0308]: mismatched types
  --> src/main.rs:25:22
   |
25 |     consumer.consume(());
   |                      ^^ expected associated type, found `()`
   |
   = note: expected associated type `<() as Factory>::Product`
                    found unit type `()`

The problem exists on stable 1.42 and nightly 1.44

[rodrimati1992]

It somehow compiles if you replace the associated type with a type parameter:

fn make_product_consumer<T,F: Factory<Product=T>>(f: F) -> impl ProductConsumer<T> {
    ()
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=44455a7c12a77aa33c75b2c95658ebdb

[jonas-schievink]

cc #60414 / #62221

[jonas-schievink]

Duh, I forgot how opaque types work.

Not sure where <opaque>: ProductConsumer<<() as Factory>::Product> would be stored (ParamEnv? It's empty at this point according to log), but it's definitely not in the type, so this output makes sense.

[benjaminp]

This is fixed on nightly.