What version of Go are you using (go version)?
$ go version
1.18
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
GO111MODULE="auto"
GOARCH="arm64"
GOBIN=""
GOCACHE="/Users/changkun/Library/Caches/go-build"
GOENV="/Users/changkun/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/changkun/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/changkun/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/changkun/goes/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/changkun/goes/go/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.18"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/changkun/dev/poly.red/polyred/go.mod"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/g2/6fmr1qzx0ns3shq74zrp6bd40000gn/T/go-build1836918707=/tmp/go-build -gno-record-gcc-switches -fno-common"
What did you do?
https://go.dev/play/p/bW8xjgG0x6Z
In this example, When we use Option1[T] (constrainted by A or B) as
an argument of NewA[T], there is only one possibility to infer the concrete
type of Option1[T], because NewA only accept A as type parameter.
package main
import "fmt"
type A struct{ val int }
type B struct{ val int }
type Option[T A | B] func(v *T)
func NewA[T A](opts ...Option[T]) *T {
a := new(T)
for _, opt := range opts {
opt(a)
}
return a
}
func NewB[T B](opts ...Option[T]) *T {
b := new(T)
for _, opt := range opts {
opt(b)
}
return b
}
func Option1[T A | B](val int) Option[T] {
return func(v *T) {
switch vv := any(v).(type) {
case *A:
vv.val = val
case *B:
vv.val = val
}
}
}
func main() {
u := NewA(Option1[A](42)) // OK
v := NewB(Option1[B](42)) // OK
// NewA(Option1(42)) // ERROR: cannot infer T
// NewB(Option1(42)) // ERROR: cannot infer T
fmt.Println(u, v) // &{42} &{42}
}What did you expect to see?
It is possible to write code without specifying type parameter in this case:
What did you see instead?
Compile error: cannot infer T
More reasons
This is a quite simplification when the options are distributed in a different package, say pkgname:
package pkgname
import "fmt"
type A struct{ val int }
type B struct{ val int }
type Option[T A | B] func(v *T)
func NewA[T A](opts ...Option[T]) *T {
a := new(T)
for _, opt := range opts {
opt(a)
}
return a
}
func NewB[T B](opts ...Option[T]) *T {
b := new(T)
for _, opt := range opts {
opt(b)
}
return b
}
func Option1[T A | B](val int) Option[T] {
return func(v *T) {
switch vv := any(v).(type) {
case *A:
vv.val = val
case *B:
vv.val = val
}
}
}
---
package main
func main() {
pkgname.NewA(pkgname.Option1[pkgname.A](42)) // Feels stutter
pkgname.NewB(pkgname.Option1[pkgname.B](42))
// pkgname.NewA(pkgname.Option1(42)) // Would be much simpler in terms of use and readability
// pkgname.NewB(pkgname.Option1(42))
}
What version of Go are you using (
go version)?Does this issue reproduce with the latest release?
Yes
What operating system and processor architecture are you using (
go env)?go envOutputWhat did you do?
https://go.dev/play/p/bW8xjgG0x6Z
In this example, When we use
Option1[T](constrainted byAorB) asan argument of
NewA[T], there is only one possibility to infer the concretetype of
Option1[T], becauseNewAonly acceptAas type parameter.What did you expect to see?
It is possible to write code without specifying type parameter in this case:
What did you see instead?
Compile error: cannot infer T
More reasons
This is a quite simplification when the options are distributed in a different package, say
pkgname: