Relevant code is here in cmd/compile/internal/gc/typecheck.go:

		if n.Left.Op == OTYPE {
			if !looktypedot(n, t, 0) {
				if looktypedot(n, t, 1) {
					yyerror("%v undefined (cannot refer to unexported method %v)", n, n.Sym)
				} else {
					yyerror("%v undefined (type %v has no method %v)", n, t, n.Sym)
				}
				n.Type = nil
				return n
			}

So maybe the problem is in the representation of n.Sym? Also fine with this being closed as "this is expected behavior."

CL https://golang.org/cl/44070 mentions this issue.

@kevinburke I've taken a stab at this - feel free to review the CL if you're at all familiar with cmd/compile :)

Also, FYI, you were slightly off in where the error actually occurs. The place is:

case lookdot(n, t, 1) != nil:
        // Field or method matches by name, but it is not exported.
        yyerror("%v undefined (cannot refer to unexported field or method %v)", n, n.Sym)

But I believe this doesn't matter, as the problem is that n.Sym.String() is what includes the weird ."" part. My CL above changes the contents of n.Sym.