[generics] go vet -generic flag

[Gert]

As far as I can tell there are already a lot of closed working as intended tickets related to

https://go.googlesource.com/proposal/+/refs/heads/master/design/go2draft-type-parameters.md#using-generic-types-as-unnamed-function-parameter-types

My suggestion is to be more proactive about this specific problem as soon as consensus is reached by introducing a `go vet -generic` flag that does nothing more then just highlight this specific very rare problem. And introduce this small language change now so it's more than a year in advance in place.

The concern I think will come forward is when we don't give it a high enough priority early enough it will significantly handicap the generic error compiler messages if we don't be concrete about it?

[Ian Lance Taylor]

That sounds like a good plan, if we decide to proceed with this syntax. Thanks.

Ian