Templates: Array slices not recognized

[Max Klyga via Digitalmars-d-learn]

On 2015-04-18 13:46:19 +0000, Chris said:

> The following:
> 
> import std.stdio : writefln;
> import std.range.primitives : isInputRange, hasLength;
> 
> void main() {
>    size_t[] a = [1, 2, 3, 4, 5, 6, 7, 8, 9];
>    doSomething(a);  // works
> 
>    doSomething(a[0..5]);
> 
> // ---> Error: template slices.doSomething cannot deduce function from 
> argument types !()(ulong[]), candidates are:
>    // slices.d(11): slices.doSomething(R)(ref R r) if (isInputRange!R 
> && hasLength!R)
> 
>    doSomething!(size_t[])(a[0..5]);
>    // ---> Error: doSomething (ref ulong[] r) is not callable using 
> argument types (ulong[])
> }
> 
> void doSomething(R)(ref R r)
>                  if (isInputRange!R && hasLength!R)  // etc..
> {
>    foreach (ref n; r) {
>      writefln("%d * 2 = %d", n, n * 2);
>    }
> }
> 
> //EOF
> 
> a[0..5] is not recognized as size_t[]. If I give the compiler a hint 
> with !(size_t[]), it complains again, i.e. I can not pass the slice as 
> a reference.
> 
> A workaround is
> 
> size_t[] b = a[0..5];
> doSomething(b);
> 
> However, this comes with a serious performance penalty in for loops 
> (even if I predefine b and reuse it in the loop). 
> to!(size_t[])(a[0..5]) is even worse.
> 
> Any thoughts or tips?

a[0..5] is an R-value, and cannot be passed by reference.
As you noticed, once you use a variable - everything works because only 
L-values can be passed by reference.

Also why are you passing slices by reference? Slices do not copy the 
memory they point to when passed by value.