为什么我必须切片数组以将其用作别名参数中的范围？

Question

通常,我不必将数组切片以将其视为范围.但是,当它作为模板别名参数传递时,我似乎这样做:

enum test(alias fun) = "it works";

immutable a = [1,2,3];

pragma(msg, test!(a[].map!(x => x))); // OK
pragma(msg, test!(a.map!(x => x)));   // map is not a member of a

// but a.map does work at runtime ...
unittest {
    auto r = a.map!(x => x);
}

为什么是这样？