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E0122: clarify wording #43176

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Aug 11, 2017
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E0122: clarify wording #43176

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ba53f95
E0122: clarify wording
RalfJung Jul 12, 2017
5067ef2
fix line lengths
RalfJung Jul 12, 2017
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6 changes: 3 additions & 3 deletions src/librustc_typeck/diagnostics.rs
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Expand Up @@ -1611,9 +1611,9 @@ static BAR: _ = "test"; // error, explicitly write out the type instead
"##,

E0122: r##"
An attempt was made to add a generic constraint to a type alias. While Rust will
allow this with a warning, it will not currently enforce the constraint.
Consider the example below:
An attempt was made to add a generic constraint to a type alias. This constraint
is entirely ignored. For backwards compatibility, Rust still allows this with a
warning. Consider the example below:
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This is incorrect. The constraint is required to use associated type syntax (without writing <T as Trait>::Assoc - which you can't for Fn traits on stable).

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Could you elaborate? Are you saying there is a difference between the constraint being "not enforced" and "entirely ignored"?

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Yes, the constraint is only ignored when referencing the alias, not within it. Actually I think <T as Trait>::Assoc still requires a T: Trait bound. And the only reason that works is we effectively recover a T: Trait bound from the presence of a <T as Trait>::Assoc type, even if we don't know whether the type came from somewhere which already required T: Trait.

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the constraint is only ignored when referencing the alias, not within it.

Are you sure?

This is accepted:

struct Test<T: Copy> { t : T}

type MyTest<T> = Test<T>;

And so it this:

trait Test {
    type Type;
}

type MyTest<T> = <T as Test>::Type;

To me it looks like constraints are not at all checked or enforced inside aliases.

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@nikomatsakis Do we not do any WF-checking in type aliases?

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@eddyb at present, I think we do not! I expect we are going to make various changes here -- possibly in a "new epoch", but hopefully we can pull it off without that.


```
trait Foo{}
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