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cmd/compile: instantiation cycle in closure function created by method of generic type #50215
Description
What version of Go are you using (go version)?
$ go version go version devel go1.18-9d0ca262bb Wed Dec 15 00:33:55 2021 +0000 darwin/amd6
Does this issue reproduce with the latest release?
Yes.
What operating system and processor architecture are you using (go env)?
go env Output
$ go env
What did you do?
https://go.dev/play/p/l9gzFUOBT5W?v=gotip
package main
import (
"fmt"
"strconv"
"strings"
)
type StringParser[T any] func(s string) (T, error)
func ParseList[T any](p StringParser[T], s string) ([]T, error) {
if s == "" {
return nil, nil
}
parts := strings.Split(s, ",")
vals := make([]T, len(parts))
for i, part := range parts {
v, err := p(part)
if err != nil {
return nil, err
}
vals[i] = v
}
return vals, nil
}
func (p StringParser[T]) ToListParser() StringParser[[]T] {
return func(s string) ([]T, error) {
return ParseList(p, s)
}
}
func ToListParser[T any](p StringParser[T]) StringParser[[]T] {
return func(s string) ([]T, error) {
return ParseList(p, s)
}
}
func main() {
p := StringParser[int](strconv.Atoi).ToListParser()
fmt.Println(p("1,2,3"))
}ToListParser is fine to compile,, but the compiler reports errors on StringParser.ToListParser, :
./prog.go:9:19: instantiation cycle:
prog.go:27:54: T instantiated as []T
as i can see, []T is depends on T, rather than StringParser[T], there shouldn't be a cycle.
I don't know whether this is a bug, or go doesn't allow this usage ?
Activity
randall77 commentedon Dec 16, 2021
@mdempsky
mdempsky commentedon Dec 16, 2021
I think the error is correct.
StringParser[T]'sToListParsermethod has a return type ofStringParser[[]T]. This means if we want to instantiateStringParser[int], then we'll also have to instantiateStringParser[[]int]too; which then means we'll also have to instantiateStringParser[[][]int], etc etc.zhuah commentedon Dec 17, 2021
@mdempsky thank you, i got it.
it' may better be reported as infinite instantiation instead of cycle ?
typesanitizer commentedon Jan 11, 2022
Could you explain how this follows from the previous part? There are no method calls for
StringParser[[]int], so it looks like there should never be any need to considerStringParser[[][]int].Similar code in (say) Rust compiles just fine:
mdempsky commentedon Jan 11, 2022
Go allows method calls to happen dynamically through interface assertions or reflection. As a result, if a type is instantiated, all of its methods must be instantiated too, even if not statically called.
There are certainly cases where we could determine this isn't actually needed, but that would complicate the language specification.