alias this inside named mixin template

[dlangBugzillaToGithub]

Simen Kjaeraas reported this on 2018-03-10T16:59:36Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=18588

Description

struct S {
    mixin foo!() bar;
}

template foo() {
    int n;
    alias n this;
}

unittest {
    int n = S.init;
}

The alias this is inside a named member of S, and it is counterintuitive that it affects S.