The variable a is of type inter, an interface, which is pointing to an impl. The struct impl has the inter interface embedded in it. That means impl.aaa() exists, which actually means impl.inter.aaa(). However in your code impl.inter is nil. To make it more clear:
b:=impl{}
var a inter
a=b
a.aaa() // this will call b.inter.aaa(), but b.inter is nil
