Use the keyword `impls` instead of `is` when writing a `where` constraint that a type variable needs to implement an interface or named constraint.
What was previously (provisionally) written:
```
fn Sort[T:! Container where .ElementType is Ordered](x: T*);
```
will now be written:
```
fn Sort[T:! Container where .ElementType impls Ordered](x: T*);
```
---------
Co-authored-by: Geoff Romer <gromer@google.com>
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>
This proposal addresses the question of whether we should perform a fully top-down compilation (like in C), a mostly top-down compilation (like in C++), or whether we should allow information from later in the same source file to be used in earlier program constructs (like in Rust, Swift, Java, C#, Haskell, and so on).
The proposed direction is:
- Entities declared later in the same source file cannot be used earlier; top-down semantics apply everywhere.
- As an exception, class member function bodies are parsed as if they appeared after the class.
- Forward declarations can be used to separate interface from implementation and to allow entities to be used before they are defined.
- The behavior of the program is nonetheless required to be the same as if we had a globally-consistent rule: it's always a hard error to depend on any information that is not known or that is provided later.
Richard Smith