Trait Constraints Are Not Respected

[IGI-111]

CS-FSSA-014

In Sway similarly to Rust, one can define function over a generic type which can be further constrained by
a trait. An example can be found in the following snippet:

trait MyTrait1{
 fn foo();
}
fn bar<T>() -> Option<T> where T: MyTrait1{
 None
}

Here bar, returns a generic Option T where T should implement MyTrait. When we force the result to
monomorphize with a type ascription, the compiler should check if the concrete type satisfies the trait
bounds. However, this doesn't happen therefore the example below successfully compiles.

fn main(){
 let x: Option<u32> = bar();
}

Note, however, that the following, seemingly equivalent snippet, will be rejected by the compiler:

fn main(){
 let x = bar::<Option<u32>>();
}

[ironcev]

#6490 fixes this issue.

We still do have a separate bug in this example. The type mentioned in the trait not implemented error is not always fully correct. This will be solved as a separate issue #6496.