New Issue: CG: visibility and applicability of 'implements' declarations

Notifications Issues
1

16731, “vasslitvinov”, “CG: visibility and applicability of ‘implements’ declarations”, “2020-11-17T20:03:22Z”

Main issue: #8629
Related: #16688
Split out from #16411

In summary, how does an ‘implements’ declaration behave?

  • (function-like) It is visible uniformly throughout the scope it is in; brought in by use statements; can be ambiguous.

  • (var-like) It is visible only in the part of the scope that follows the declaration; can be brought in by use statements; shadows similar declarations in enclosing scopes.

  • (something else) ?

Here are a couple of specific aspects, moved here from #16411:

Importability of implements declarations

When a module contains implements declarations, are they brought in with a use or import? For example:

module Impl {
  implements X(int);
}
use Impl;
use Impl except X;
use Impl only Y;
import Impl;
// is `implements X(int)` visible here upon each of the above?

Under both (function-like) and (var-like), implements declarations are use-able and can be restricted with except and only.

What should happen upon import Impl; ?

Shadowing of implements declarations

What happens with two implements X(int), in an outer scope and an inner scope? If my call to a CG function requires implements X(int), is this an ambiguity or the closer statement wins?

{
  implements X(int);
  {
    implements X(int);
    {
      ...querying implements X(int)... // OK or "ambiguous" ?
    }
  }
}

(var-like): the inner statement shadows the outer statement.
(function-like) the two statements are ambiguous.

If there are two implements X(int) in the same scope, they are clearly ambiguous.

implements applicability to textually-preceding code

(discussed further in #16688)

Should an implements statement be visible:
(var-like) only in textually-following code, or
(function-like) anywhere in the scope where it occurs?

For example:

proc x(arg) where implements X(arg.type) {...}
x(5); // OK if "function-like", error if "var-like"
implements X(int);