type alias with type that have type parameters doesn't work in go1.18rc

[adamluzsi]

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

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

Does this issue reproduce with the latest release?

go1.18rc1 seems to be the latest at the moment. (Mon Mar 7 09:50:11 AM CET 2022)

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/adamluzsi/.cache/go-build"
GOENV="/home/adamluzsi/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/adamluzsi/pkg/mod"
GONOPROXY="*"
GONOSUMDB="*"
GOOS="linux"
GOPATH="/home/adamluzsi"
GOPRIVATE="*"
GOPROXY="direct"
GOROOT="/home/adamluzsi/dist/golang.org/1.18rc"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/adamluzsi/dist/golang.org/1.18rc/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.18rc1"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/adamluzsi/src/github.com/adamluzsi/frameless/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 -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build4186401866=/tmp/go-build -gno-record-gcc-switches"

What did you do?

package bug

import "io"

type Iterator[T any] interface {
	// Closer is required to make it able to cancel iterators where resource being used behind the scene
	// for all other case where the underling io is handled on higher level, it should simply return nil
	io.Closer
	// Next will ensure that Decode return the next item when it is executed
	Next() bool
	// Err return the cause if for some reason by default the More return false all the time
	Err() error
	// Value returns the current value from the iterator.
	Value() T
}

// fails with:
//   ./bug.go:19:22: cannot use generic type Iterator[T any] without instantiation
type IteratorAlias = Iterator

What did you expect to see?

Based on the following document guidance: go.googlesource.com/proposal#generic-type-aliases
I hoped that I can make a type alias for a type that has type parameters with refactoring purposes.

What did you see instead?

./bug.go:19:22: cannot use generic type Iterator[T any] without instantiation

Let me know if I can help with the issue,
and thank you very much for your support!

[mengzhuo]

Thanks for the bug report.
As the error message says, this is a dup of #25838, #50729, #50925.
Closing as a dup.

[adamluzsi]

I fully trust your judgment @mengzhuo that those are related to this and not a type parameters related issue.
I wish you a great day! : )

[gofish]

@mengzhuo are you certain this is a dup? This is not an issue of parameterized aliases, nor or type alias cycles. A trivial type alias to any parameterized type is rejected, contrary to the example in the generics proposal itself: https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#generic-type-aliases

Asking mostly because that "umbrella" issue sounds larger in scope and because a similar issue appears to have cropped up in the generics prototype, which @griesemer fixed easily there: #39768

https://go.dev/play/p/yC68DoFSKrl

package main

type Vector[T any] []T

type VectorAlias = Vector

var v VectorAlias[int]

./prog.go:5:20: cannot use generic type Vector[T any] without instantiation

Otherwise please ignore me and I will follow #25838.

[gofish]

This is not an issue of parameterized aliases

Unless of course permitting an alias to a parameterized type implies a parameterized alias which could be subject to cycle issues? I'm new to go1.18 generics and am learning, thanks!

[griesemer]

Per the spec:

If the type definition specifies type parameters, the type name denotes a generic type. Generic types must be instantiated when they are used.

Iterator is a generic type, it must be instantiated when used, including when used on the RHS of alias declaration.

This is working as intended.