Works
---
mixin template foo(bool b)
{
    int _impl() { writeln(b); return int.init; }
    int _ipml2 = _impl();
}
void main()
{
  mixin foo;
}
---
doesn't
---
mixin template foo(bool b)
{
        writeln(b);
}

void main()
{
  mixin foo;
}
---

see https://forum.dlang.org/post/txcblywfkrhpxnsbfydk@forum.dlang.org for the discovery of this idiom