What version of Go are you using (go version)?

go version devel +6b6b9f69fd Wed Aug 9 05:33:09 2017 +0000 linux/amd64
go version go1.9 linux/amd64
go version go1.8.3 linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

https://play.golang.org/p/Vne0Zww-Qf

What did you expect to see?

package gps ("qps") <nil>

What did you see instead?

package gps ("qps") main.go:37:20: cannot use UnpairedVersion(nil) (value of type UnpairedVersion) as Constraint value in variable declaration: missing method copyTo

The Go compiler accepts this code. The example is based on a (much, much larger) real code base (golang/dep#1127)

/cc @griesemer