The following code is accepted by both gc and gccgo, but not go/types:

func foo(args ...string) {}

func bar() {
  foo(nil...)
}

The error returned by go/types is

cannot use nil (untyped nil value) as parameter of type []string

It appears that go/types is wrong here. Per the spec
( https://golang.org/ref/spec#Passing_arguments_to_..._parameters, https://golang.org/ref/spec#Assignability ):

If f is variadic with a final parameter p of type ...T, then within f the type of p is equivalent to type []T.
...
If the final argument is assignable to a slice type []T, it may be passed unchanged as the value for a ...T parameter if the argument is followed by .... In this case no new slice is created.

A value x is assignable to a variable of type T ("x is assignable to T") in any of these cases:
...

• x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type.