unsafe: Alignof, Offsetof, Sizeof documentation does not account for variable-size types

[zephyrtronium]

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 (master)

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/zephyr/.cache/go-build"
GOENV="/home/zephyr/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/zephyr/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/zephyr/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.18"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
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-build1655306045=/tmp/go-build -gno-record-gcc-switches"

What did you do?

go doc unsafe.Sizeof

What did you expect to see?

A statement that variables whose types are or contain type parameters have variable size, and so Alignof, Offsetof, and Sizeof are not constant expressions in those cases.

What did you see instead?

The return value of Sizeof is a Go constant.

This implies that https://go.dev/play/p/ZcHTDWGcQgG should compile, but it does not.

Partially related: #40322

@gopherbot please add label Documentation

[gopherbot]

Change https://go.dev/cl/417917 mentions this issue: unsafe: document when Sizeof/Offsetof/Alignof are not constant