Allow casting of AliasSeq

[dlangBugzillaToGithub]

Simen Kjaeraas reported this on 2017-09-07T13:50:43Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=17815

Description

The following code does not currently compile:

    import std.meta : AliasSeq;
    AliasSeq!(int, float) a;
    auto b = cast(AliasSeq!(byte, byte))a;

Casting a set of values to a set of types is something that comes up every now and then. It can currently be done using a variation of this function:

    template castTuple(T...) {
        auto castTuple(Args...)(Args args) if (Args.length == T.length) {
            static if (T.length == 0) {
                return tuple();
            } else {
                auto result = .castTuple!(T[1..$])(args[1..$]);
                return tuple(cast(T[0])args[0], result.expand);
            }
        }
    }

The compiler however, has all the necessary information to make the first example compile.