env TESTGO_VERSION=go1.21
! go mod download rsc.io/future@v1.0.0
stderr '^go: rsc.io/future@v1.0.0 requires go >= 1.999 \(running go 1.21\)$'

-- go.mod --
module m
go 1.21

-- x.go --
package p

import "rsc.io/future/foo"