Hi,
I'm not a generics expert either, but I think there is a misunderstanding
here:
The A in the Wrap[A any] is not the same as the A in Val[A any]. When you
instantiate the Extract function with Extract[string], it expects a
Wrap[string], which is, in fact, just a type implementing isWrap. The Val[A
any] implements this method whatever the type it is instantiated with
(string, int...) so Extract[string](Val[int]{3}) is, from my understanding,
a completely valid expression. The only thing we can be certain of with
this code is that the type switch will **not** reach the default case only
precisely when the Val[A any] is instantiated with the same type as
Extract[A any] function.
Hope I'm not getting anything wrong :)
On Tue, Jun 21, 2022 at 12:27 AM Tsvi Benschar <tsvihbensc...@gmail.com>
wrote:
> Hey everyone, reposting a question
> <https://forum.golangbridge.org/t/generic-interface-compiles-with-incorrectly-instantiated-types/27809>
> I
> asked on the go forum since it isn't getting replies there. I have some
> code that’s compiling and running despite there being a type error. I don’t
> understand go generics and interfaces that well, so I don’t know if this
> behavior is intended.
>
> Here's a simple incorrect program <https://go.dev/play/p/UpEffRk8oB9> that
> compiles. (Here's the correct version <https://go.dev/play/p/pE9AWb-xvda>.)
> On line 23 of the incorrect version, the function's generic parameter is
> instantiated to string, but its input is of type int. It compiles anyways
> and panics at runtime, even though the type switch's default case should
> never be reached.
>
> I got some similar examples to compile and run, they all involve
> instantiating a generic interface with one type but using it as if it had a
> different type. Is this behavior intended, or maybe is there some implicit
> unsafe cast happening? Or is this a bug?
>
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