Add support implicit conversion between types

[monarch_dodra]

On Friday, 6 September 2013 at 13:50:25 UTC, ilya-stromberg wrote:
> On Friday, 6 September 2013 at 13:30:26 UTC, Dicebot wrote:
>> So, what essentially is needed, is ability to implicitly 
>> convert literals of built-in types to user types, not any 
>> possible implicit conversion?
>>
>> I think allowing it with such restrictions can be reasonably 
>> clean.
>
> Yes, the ability to implicitly convert literals of built-in 
> types to user types is REALLY needed.
> The 2-nd error from "factorial" example
>
> import std.bigint;
>
> void main()
> {
>    assert(factorial!BigInt(BigInt(3)) == 6); //Error: 
> incompatible types for ((1) : 
> (number.opBinary(factorial(number.opBinary(1))))): 'int' and 
> 'BigInt'
> }
>
>
> The correct factorial implementation is:
>
> T factorial(T)(T number)
> {
>    enum T one = 1;
>    return number <= one ? one : number * factorial!T(number - 
> one);
> }

Or just:

//----
import std.bigint;

T factorial(T)(T number)
{
     return number <= 1 ?
         T(1) :
         number * factorial!T(number - 1);
}
//----

The problem though is that this requires "uniform construction", 
which we don't have yet:
http://d.puremagic.com/issues/show_bug.cgi?id=9112