TypeFunction example: ImplictConvTargets

[Paul Backus]

On Monday, 5 October 2020 at 20:57:04 UTC, Stefan Koch wrote:
> On Monday, 5 October 2020 at 12:50:39 UTC, Andrei Alexandrescu 
> wrote:
>
>> The existing implementation is ancient (e.g. predates 
>> std.meta) and certainly deserves a redoing with Filter, which 
>> turns it into a 3-liner (untested):
>>
>> alias Integrals = AliasSeq!(byte, ubyte, short, ushort, int, 
>> uint, long, ulong, CentTypeList, float, double, real, char, 
>> wchar, dchar);
>> alias convertsTo(U) = is(T : U);
>> alias ImplicitConversionTargets(T) = Filter!(convertsTo, 
>> Integrals);
>
> This code does not work.
> I don't even need to compile it to see that.

It has some simple mistakes, but the fundamental idea is sound. 
Here's a version that actually compiles:

import std.meta;

alias Numerics = AliasSeq!(byte, ubyte, short, ushort, int, uint, 
long, ulong, float, double, real, char, wchar, dchar);
enum convertsTo(T, U) = is(T : U);
alias ImplicitConversionTargets(T) = 
Filter!(ApplyLeft!(convertsTo, T), Numerics);

// prints: (int, uint, long, ulong, float, double, real, dchar)
pragma(msg, ImplicitConversionTargets!int);
// prints: (float, double, real)
pragma(msg, ImplicitConversionTargets!double);