env GO111MODULE=on

# Download modules to avoid stderr chatter
go mod download example.com@v1.0.0
go mod download example.com/newcycle/a@v1.0.0
go mod download example.com/newcycle/a@v1.0.1
go mod download example.com/newcycle/b@v1.0.0

go mod init m
! go get example.com/newcycle/a@v1.0.0
cmp stderr stderr-expected

-- stderr-expected --
go: example.com/newcycle/a@v1.0.0 requires example.com/newcycle/a@v1.0.1, not example.com/newcycle/a@v1.0.0