Allow forward referencing IFTI types in template argument list

[dlangBugzillaToGithub]

Vladimir Panteleev (@CyberShadow) reported this on 2014-03-21T11:17:57Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=12433

CC List

• Seb

Description

Consider this hypothetical example:

///////////////// test.d /////////////////
T[] copyArray(T=I, I)(I[] input)
{
    auto result = new T[input.length];
    foreach (n, ref i; input)
        result[n] = i;
    return result;
}

void main()
{
    // copy as is
    int[] r1 = copyArray([1, 2, 3]);
    // copy to another type
    long[] r2 = copyArray!long([1, 2, 3]);
}
//////////////////////////////////////////

There is currently no way to get this code to work, without either declaring an overload for copyArray, or replacing T=I with T=SomeDummyType and later checking to see if something was explicitly passed or not.

[dlangBugzillaToGithub]

hsteoh commented on 2014-09-25T20:26:19Z

Related: issue #10228

[dlangBugzillaToGithub]

greensunny12 commented on 2016-05-06T11:01:10Z

It's not an "hypothetical example" - it makes real code ugly! :/

Let me add an example from a recent PR in phobos
https://github.com/dlang/phobos/pull/4263

If we could support the forwarding of argument types, we would only have _one_, very nice & readable function header:


```
T[] makeArray(T = Unqual!(ElementType!R), Allocator, R)
```


Of course we can add another function overload, but it unnecessary bloats!

```
Unqual!(ElementType!R)[] makeArray(Allocator, R)(auto ref Allocator alloc, R range)
{
      import std.range.primitives;
      alias T = Unqual!(ElementType!R);
      return makeArray!(T, Allocator, R)(alloc, range);
}

T[] makeArray(T, Allocator, R)(auto ref Allocator alloc, R range)	  
```