Generics: cannot use common struct fields of a type set.

[beoran]

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

$ go version
/tmp/golang-tip/bin/go version                                                                                                                      
go version devel go1.18-986f8ea6b4 Tue Sep 21 00:59:42 2021 +0000 linux/amd64

Does this issue reproduce with the latest release?

No, it is a generics issue, therefore tested with a recent tip only.

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

linux/amd64

go env Output

$ go env
GO111MODULE="" 
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/me/.cache/go-build"
GOENV="/home/me/.config/go/env"            
GOEXE=""                                                                                                                                                                             GOEXPERIMENT=""                                                                                                                                                                      GOFLAGS=""                                                                                                                                                                           GOHOSTARCH="amd64"                                                                                                                                                                   GOHOSTOS="linux"                                                                                                                                                                     GOINSECURE=""                                                                                                                                                                        GOMODCACHE="/home/me/src/go/pkg/mod"                                                                                                                                           GONOPROXY="k8s.io/*"
GONOSUMDB=""                                                                                                                                                                         GOOS="linux"                                                                                                                                                                         GOPATH="/home/me/src/go"                                                                                                                                                       GOPRIVATE=""                                                                                                                                                                         GOPROXY=""                                                                                                                                 
GOROOT="/tmp/golang-tip"                                                                                                                                                             GOSUMDB="off"                                                                                                                                                                        GOTMPDIR=""                                                                                                                                                                          GOTOOLDIR="/tmp/golang-tip/pkg/tool/linux_amd64"                                                                                                                                     GOVCS=""                                                                                                                                                                             GOVERSION="devel go1.18-986f8ea6b4 Tue Sep 21 00:59:42 2021 +0000"                                                                                                                   GCCGO="gccgo" 
GOAMD64="v1"                                                                                                                                                                         AR="ar"                                                                                                                                                                              
CC="gcc"                                                                                                                                                                             CXX="g++"                                                                                                                                                                            CGO_ENABLED="1"                                                                                                                                                                      GOMOD="/home/me/src/gocrtp/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-build1474680903=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I tried to compile this program (crtp.go) with generics:

package main

import "fmt"

type Point struct {
	X, Y int
}

type Rect struct {
	X, Y, W, H int
}

type Elli struct {
	X, Y, W, H int
}

func GetX[P interface { Point | Rect | Elli }] (p P) int {
	return p.X
}

func main() {
	p := Point { 1, 2}
	r := Rect {2, 3, 7, 8}
	e := Elli {4, 5, 9, 10}
	fmt.Printf("X: %d %d %d\n", GetX(p), GetX(r), GetX(e))
}

with tmp/golang-tip/bin/go build

What did you expect to see?

Program compiles, runs and outputs X: 1 2 4

What did you see instead?

./crtp.go:19:11: p.X undefined (type bound for P has no method X)

All three structs in the type bound have an identical X /field/, so I think this is wrong. Of course there is no method but I don't think that matters here. I feel I should be able to use the public field X of p since p can only be one of the three Point, Rect, or Elli.

[beoran]

@gopherbot, please add label generics

[dr2chase]

@griesemer

Reading "Composite types in constraints" in https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md I get the impression that this ought to work; the example there says "this doesn't work because the field types for X don't all match" which implies that if the field types did match, as they do here, then it would work.

package main

import "fmt"

type hasX interface {
	struct {
		X, Y int
	} | struct {
		X, Y, H int
	} | struct {
		X, Y, W, H int
	}
}

func GetX[P hasX](p *P) int {
	return p.X
}

func main() {
	p := struct {
		X, Y int
	}{1, 2}
	r := struct {
		X, Y, H int
	}{2, 3, 8}
	e := struct {
		X, Y, W, H int
	}{4, 5, 9, 10}
	fmt.Printf("X: %d %d %d\n", GetX(&p), GetX(&r), GetX(&e))
}

[griesemer]

We don't currently support field accesses of this kind even though the proposal says that this should/could work. We may not support this for Go1.18 as it doesn't seem like an essential feature. There's a trivial work-around that uses a method:

package main

import "fmt"

type Point struct {
	X, Y int
}

func (p Point) GetX() int { return p.X }

type Rect struct {
	X, Y, W, H int
}

func (r Rect) GetX() int { return r.X }

type Elli struct {
	X, Y, W, H int
}

func (e Elli) GetX() int { return e.X }


func GetX[P interface { Point | Rect | Elli; GetX() int }] (p P) int {
	return p.GetX()
}

func main() {
	p := Point { 1, 2}
	r := Rect {2, 3, 7, 8}
	e := Elli {4, 5, 9, 10}
	fmt.Printf("X: %d %d %d\n", GetX(p), GetX(r), GetX(e))
}

Of course, then you don't need generic code in the first place because you could just use dynamic method dispatch. And maybe you should.

[beoran]

The reason I tried this out is for #48499, wherr the OP directly wants to get fields or pointers to fields from a set of similar structs. In that use case, the overhead of an accessor function and dynamic method dispatch would be not acceptable.

So while it may not be essential, #48499, which is for use with Go language databases, goes to show that it is not academic, but actually would be very useful for existing code, in stead of the feature proposed in that issue. Furthermore it is more consistent and easier to learn to also allow it.

If there is not enough time left to implement this for 1.18, then please consider this for 1.19.