> The Wrap interface ignores its type parameter Curious, why doesn't the compiler consider this an error? I would expect a "type parameter declared but not used" error, similar to the "variable declared but not used" error. Or are there valid use cases for types that declare a type parameter but do not use it?
On Tuesday, June 21, 2022 at 12:49:10 AM UTC+2 Ian Lance Taylor wrote: > On Mon, Jun 20, 2022 at 3:27 PM Tsvi Benschar <tsvihb...@gmail.com> > wrote: > > > > Hey everyone, reposting a question 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 that compiles. (Here's the correct > version.) 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? > > Thanks for providing a complete test case. > > The Wrap interface ignores its type parameter, so any type that > implements a isWrap method with no arguments and no results will > implement Wrap with any type argument. The argument to Extract is > Wrap[A], so you can pass anything with an isWrap method to Extract. > The type Val[int] does have an isWrap method, so it's fine to pass > that type to Extract[int]. > > I don't understand what your code is trying to do, but the behavior > seems entirely correct to me. > > Ian > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/aa1f0a02-7fa3-414b-8be0-456e951cb439n%40googlegroups.com.
