go/types2: Parametrized type method cannot see constraint struct fields

[dtcaciuc]

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

$ go version
go version go1.18 linux/amd64

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
GOHOSTARCH="amd64"
GOHOSTOS="linux"

What did you do?

The following compiled with go1.18rc1 but not with go1.18 final.

I am not sure if this was supposed to work like that or this was a bug in RC1 that was fixed, but it is useful when Handler is being used as a parameter to another type (say, Scheduler), which needs to call Handler with Work values. EDIT: seeing "has no field or" message tells me that this should be working.

package ticket

import (
        "fmt"
)

type Work struct {
        V int
}

type Handler[T Work] struct {
}

func (h *Handler[T]) Run(work T) {
        fmt.Print(work.V)
}

What did you expect to see?

Successful compilation

$ go version
go version go1.18rc1 linux/amd64
$ go build x.go
$

What did you see instead?

$ go version
go version go1.18 linux/amd64
$ go build x.go
# command-line-arguments
./x.go:15:17: work.V undefined (type T has no field or method V)
$

[findleyr]

See #51576: field access on type parameters was removed for 1.18. It may be added back in a later version of Go.

CC @griesemer -- do we have a tracking issue/proposal for deciding whether to allow this code?

[seankhliao]

This one? #51259

[findleyr]

#51259 is an even more sophisticated feature, but yes perhaps we can consolidate discussion there.

In any case, closing this as WAI, for now. Please reopen if I'm missing something.