Generics: cannot union types with behavioral interfaces

[extemporalgenome]

What version of Go are you using (go version)?

$ go version
go version devel go1.18-eba0e866fa Mon Oct 18 22:56:07 2021 +0000 linux/amd64

Does this issue reproduce with the latest release?

N/A: tip

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN="/home/kevin/usr/bin"
GOCACHE="/home/kevin/.cache/go-build"
GOENV="/home/kevin/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/kevin/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/kevin/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/kevin/goroot"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/kevin/goroot/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="devel go1.18-eba0e866fa Mon Oct 18 22:56:07 2021 +0000"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/kevin/src/sandbox/go2/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build3965098965=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I attempted to make use of the Stringish example from the proposal:

package main

import (
  "fmt"
  "strconv"
)

type Stringish interface {
	string | fmt.Stringer
}

func ToString[T Stringish](v T) string {
  switch x := (interface{})(v).(type) {
  case string:
    return x

  case fmt.Stringer:
    return x.String()
  }

  panic("impossible")
}

func PrintSlice[T Stringish](lst []T) {
  for _, v := range lst {
    fmt.Println(ToString(v))
  }
}

type Int int

func (i Int) String() string {
  return strconv.Itoa(int(i))
}

func main() {
  PrintSlice([]Int{1, 2, 3})
  PrintSlice([]string{"x", "y", "z"})
}

What did you expect to see?

1
2
3
x
y
z

What did you see instead?

go run main.go
# command-line-arguments
./main.go:9:11: cannot use fmt.Stringer in union (interface contains methods)

[randall77]

We've decided not to allow interfaces in unions, for the first version of generics at least. See #45346 (comment)

[klueska]

Here is the compromise I came up with.

It keeps the code as generic as it can at the cost of:

1. Introducing a String type with underlying type string that implements fmt.Stringer in the obvious way.
2. Having two functions instead of 1 (i.e. PrintSliceI and PrintSliceP), with the function operating on primitive types (i.e. ~string) suffixed with a P and the one operating on interface types (i.e. fmt.Stringer) suffixed with an I.
3. Using an unsafe cast to turn any slices of ~string directly into slices of String (with type safety actually still being preserved because of the constraint enforced on the PrintSliceP method of only accepting ~string types and the knowledge that all such types are backed under the hood by a string type.).

https://go.dev/play/p/NaEMP4hLBhq?v=gotip

package main

import (
	"fmt"
	"strconv"
	"unsafe"
)

type Int int
type String string
type SuperString string

func (i Int) String() string {
	return strconv.Itoa(int(i))
}

func (s String) String() string {
	return string(s)
}

func (s SuperString) String() string {
	return "Super " + string(s)
}

func Cast[U any, T any](v T) U {
	return *(*U)(unsafe.Pointer(&v))
}

func PrintSliceP[U []T, T ~string](lst U) {
	PrintSliceI(Cast[[]String](lst))
}

func PrintSliceI[U []T, T fmt.Stringer](lst U) {
	for _, v := range lst {
		fmt.Println(v.String())
	}
}

func main() {
	PrintSliceI([]Int{1, 2, 3})

	PrintSliceP([]string{"x", "y", "z"})

	PrintSliceI([]String{"x", "y", "z"})
	PrintSliceP([]String{"x", "y", "z"})

	PrintSliceI([]SuperString{"x", "y", "z"})
	PrintSliceP([]SuperString{"x", "y", "z"})
}

1
2
3
x
y
z
x
y
z
x
y
z
Super x
Super y
Super z
x
y
z

Program exited.

I see this as a pattern that will likely crop up quite often.
Does anyone have a better way to do this given the current limitation pointed out by @randall77?