On Fri, Jul 3, 2020 at 3:05 PM, Ian Lance Taylor <[email protected]> wrote:
>
> Using an interface type in a type list has a defined meaning: the type
> argument must have as its underlying type that interface type. It
> doesn't mean what I think you want it to mean, which is to serve as a
> union of the type lists of the interface types. What you suggest
> would not be impossible, but it would mean making an exception to the
> rules for type lists.
One way to disambiguate would be to define a pseudo-type, I(T) for some
interface type I and type T which is identical to T if T implements I, and
is otherwise an empty type that doesn't match any other type. This could
also solve the type switching on type parameters question, since
switch t := t.(type) {
case interface { type int }(T):
...
}
would match for a value t of type T if the underlying type of T is int. The
syntax is a bit unwieldy, but it makes it clear that we're matching based
on the type-list semantics and disambiguates between matching to an
interface type vs. matching to a concrete type that implements an
interface. That it looks like a conversion shouldn't be too much of an
issue, since generic functions are also to be followed by either a type
list or a value list.
These pseudo-types would probably only make sense in type lists and type
switches, just as null is only usable as a pseudo-type in type switches.
There might be cleaner ways of doing this, though.
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