x/tools/gopls: modernize slices.Clone suggestion wrong when copying between aliased types

[anderseknert]

gopls version

v0.18.1

go env

-

What did you do?

Ran the modernize linter on this example:

package main

import "fmt"

type ssa []string
type ssb []string

var a ssa = ssa{"a", "b", "c"}

func main() {
	var b ssb = append([]string{}, a...)

	fmt.Println(b)
}

go run golang.org/x/tools/gopls/internal/analysis/modernize/cmd/modernize@latest -fix ./...

What did you see happen?

package main

import "slices"

import "fmt"

type ssa []string
type ssb []string

var a ssa = ssa{"a", "b", "c"}

func main() {
	var b ssb = slices.Clone(a)

	fmt.Println(b)
}

go build
./main.go:13:14: cannot use slices.Clone(a) (value of slice type ssa) as ssb value in variable declaration

What did you expect to see?

slices.Clone not being suggested here, or at least not in a form that breaks compilation.

The example is silly, but I encountered this in real code when trying to run modernize against the OPA project, here specifically.

Editor and settings

No response

Logs

No response

[gabyhelp]

Related Issues

• x/tools/gopls/internal/analysis/modernize: append([]T{}, x...) -> slices.Clone(x) fix may unsoundly return nil #73557

Related Code Changes

• all: use slices.Clone or bytes.Clone to simplify code

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[xieyuschen]

Similar to #73663 , I will help to send a fix.

Edit: according to go spec, If f is variadic with a final parameter p of type ...T, then within f the type of p is equivalent to type []T..

The named type ssa could be understood as []string when type checker checks function variadic argument. However, type checker has a stricter rule on slices.Clone function call, which explicitly requires a slice. As ssa is a named type, it's no longer a slice but a separated type.

[anderseknert]

@xieyuschen yeah, var b ssb = append(ssb{}, a...) works too (instead of append to []string{}), but neither can be directly converted to use slices.Clone. Explicit conversion to the common type type (or whatever []string would be called here) works, e.g.

var b ssb = slices.Clone([]string(a))

And perhaps that's the way to go if this should remain a suggestion/fix for this particular case.