Bug 17921 – allow to use `alias function this` to be used for implicit conversions

cat > enh.d << CODE struct IPv4Addr { alias IPAddr this; // alias type constructor for implicit conversion } // either IPv4 or IPv6 struct IPAddr { this(IPv4Addr) {} } void func(IPAddr) { } unittest { func(IPv4Addr()); } CODE dmd -c -unittest enh.d ---- enh.d(3): Error: IPAddr is not a member of IPv4Addr ---- At the moment alias this for implicit conversions needs a dummy member method. When you already have `IPAddr(IPv4Addr.init)` it's somewhat annoying to add `IPv4Addr.init.toIPAddr` to the public API, only to support implicit conversions. Now there are 2 ways to do the same thing, sth. that's usually to be avoided in API design. Alias-thising a free function or type would be resolved like `var.func` calls, and according to the current alias this rules. Obviously this should be coordinated with the multiple alias this work.

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