garble build main.go
exec ./main
cmp stderr main.stderr

-- main.go --
package main

import "go/ast"

var V interface{}

type T struct{
	ast.Node
}

func main() {
	switch V := V.(type) {
	case int:
		var _ int = V
	case nil:
		println("nil case")
	}
}
-- main.stderr --
nil case