unused: false positive when only use is constant expression

[peterbourgon]

$ staticcheck -version
staticcheck 2020.1.5

$ staticcheck -debug.version
staticcheck 2020.1.5

Compiled with Go version: go1.14.4
Main module:
        honnef.co/go/tools@v0.0.1-2020.1.5 (sum: h1:nI5egYTGJakVyOryqLs1cQO5dO0ksin5XXs2pspk75k=)
Dependencies:
        github.com/BurntSushi/toml@v0.3.1 (sum: h1:WXkYYl6Yr3qBf1K79EBnL4mak0OimBfB0XUf9Vl28OQ=)
        golang.org/x/tools@v0.0.0-20191130070609-6e064ea0cf2d (sum: h1:/iIZNFGxc/a7C3yWjGcnboV+Tkc7mxr+p6fDztwoxuM=)

$ go version
go version go1.15 darwin/amd64

go env provided upon request.

package main

import (
	"io"
	"unsafe"
)

var (
	w  io.Writer
	sz = unsafe.Sizeof(w)
)

func main() {
	println(sz)
}

$ go run main.go
16

$ staticcheck main.go
main.go:9:2: var w is unused (U1000)

[dgryski]

A workaround: sz = unsafe.Sizeof((io.Writer)(nil))

[kortschak]

I see a similar case that I can't see a similar work around for.

package main

import "unsafe"

type t struct {
	int
}

const s = unsafe.Sizeof(t{})

func main() {
	println(s)
}

$  staticcheck              
main.go:5:6: type t is unused (U1000)