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Explicitly point out alternatives to trait Alias = where PREDICATES; #4

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Apr 13, 2017
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Explicitly point out alternatives to trait Alias = where PREDICATES; #4

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14 changes: 14 additions & 0 deletions text/0000-trait-alias.md
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Expand Up @@ -282,6 +282,20 @@ constraints, instead of creating an alias for a set of supertraits and predicate
something like `constraint Foo<T> = T: Bar, Vec<T>: Baz;`, used as `fn quux<T>(...) where Foo<T> { ... }`
(i.e. direct substitution). Trait object usage is unclear.

## Syntax for sole `where` clause.

The current RFC specifies that it is possible to use only the `where` clause by leaving the list of traits empty:

```rust
trait DebugDefault = where Self: Debug + Default;
```

This is one of many syntaxes that are available for this construct. Alternatives include:

* `trait DebugDefault where Self: Debug + Default;` (which has been [considered and discarded](https://github.com/rust-lang/rfcs/pull/1733#issuecomment-257993316) because [it might look](https://github.com/rust-lang/rfcs/pull/1733#issuecomment-258495468) too much like a new trait definition)
* `trait DebugDefault = _ where Self: Debug + Default;` (which was [considered and then removed](https://github.com/rust-lang/rfcs/pull/1733/commits/88d3074957276c7201147fc625f18e0ebcecc1b9#diff-ae27a1a8d977f731e67823349151bed5L116) because it is [technically unnecessary](https://github.com/rust-lang/rfcs/pull/1733#issuecomment-284252196))
* `trait DebugDefault = Self where Self: Debug + Default;` (analogous to previous case but not formally discussed)

# Unresolved questions
[unresolved]: #unresolved-questions

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