Special Type Challenge

[Jonathan Marler via Digitalmars-d]

On Saturday, 7 February 2015 at 02:12:08 UTC, Jakob Ovrum wrote:
>>  Byte echo(Byte b)
>>  {
>>    return b;
>>  }
>>  b = echo('a');
>>  b = echo(cast(const char)'a');
>>  b = echo(cast(immutable char)'a');
>>  b = echo(cast(byte)1);
>>  b = echo(cast(const byte)1);
>>  b = echo(cast(immutable byte)1);
>>  b = echo(cast(ubyte)1);
>>  b = echo(cast(const ubyte)1);
>>  b = echo(cast(immutable ubyte)1);
>>
>>  Byte[] barr;
>>
>>  barr = "teststring";
>>  barr = [0,1,2,3];
>> }
>
> These are not possible as D does not support implicit 
> construction.

It's a bit odd that D supports implicit conversions from 
user-defined types to built-in types but not the reverse.  What's 
the reason for this?  Is it done on purpose or is this just a 
hole that might be filled in later?