Capabilities in union types

[sylvanc]

Capabilities are not encoded at runtime. A union of two types, with differing capabilities, but the same types otherwise, cannot at runtime "determine" what reference was assigned to it.

As a result, when pattern matching, the following should return false, not true:

var x: A ref = A(...)
var y: (A ref | A tag) = x

match y
| var z: A ref => true
else
  false
end

In other words, a union that contains the same "base type" with two different capabilities can only make the weaker guarantee.

[sylvanc]

Fixed.