Hi Ian, thanks for having a look. I'll open a bug report. On Wednesday, February 16, 2022 at 9:46:09 PM UTC+1 Ian Lance Taylor wrote:
> On Tue, Feb 15, 2022 at 2:49 PM Blackgreen <bg.the...@gmail.com> wrote: > > > > I came across this stack overflow question: > > > https://stackoverflow.com/questions/71131665/generics-pass-map-with-derived-types > > > > The OP attempted to pass different maps into this code: > > > > func equal[M1, M2 ~map[K]V, K, V comparable](m1 M1, m2 M2) bool { if > len(m1) != len(m2) { return false } for k, v1 := range m1 { if v2, ok := > m2[k]; !ok || v1 != v2 { return false } } return true } > > > > It is clear to me why this doesn't work. However, I was studying > possible fixes for this function. Here's a playground: > https://gotipplay.golang.org/p/NPpau1G0Hqp > > > > My first attempt was based on differentiating the K and V type params > and came up with this: > > > > func equal[M1 ~map[K1]V1, M2 ~map[K2]V2, K1, K2 ~uint32, V1, V2 > ~string](m1 M1, m2 M2) bool { > > if len(m1) != len(m2) { > > return false > > } > > for k, v1 := range m1 { > > if v2, ok := m2[K2(k)]; !ok || V2(v1) != v2 { > > return false > > } > > } > > return true > > } > > > > I thought this should work, because now it's possible to infer K1 and K2 > from M1 and M2 respectively, and the approximate constraints allow > conversion in the function body. But it still doesn't compile with error > "K2 does not match uint32". > > > > Instead, this works, even though K1 and K2 are defined the same way: > > > > func equalFixed[K1, K2 ~uint32, V1, V2 ~string](m1 map[K1]V1, m2 > map[K2]V2) bool { > > } > > > > By reading the Go 1.18 language specs, my intuition is that the first > attempt fails due to the so-called "adjusted core type" of the constraint > ~uint32, though I'm not 100% sure of what's going on. > > Can you folks confirm my intuition and/or provide some pointers? > > > This looks like a bug in type inference to me. I may be missing > something but I don't see how it could be producing that error. Would > you mind opening a bug report at https://go.dev/issue? Thanks. Note > that this may not be fixed in the upcoming 1.18 release, as we are > trying to be careful about type inference. > > 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/ed5cdd4f-1e1f-4a1f-9ffb-d6dbf73940e6n%40googlegroups.com.
