Reimplementing the bulk of std.meta iteratively

[Stefan Koch]

On Thursday, 1 October 2020 at 08:56:58 UTC, Walter Bright wrote:
>
> I had looked into this. Unfortunately, many aspects of types 
> are simply too complex to try and represent at run time. For 
> example, is type T1 implicitly convertible to T2? This 
> seemingly simple question is very complex. Yes, `alias this` 
> makes it much worse :-/
>


The my latest post please!
The example on type functions with the conversion matrix.

This is all you need:

     override void visit(IsExp e)
     {
         auto targe = e.targ.isTypeExpression();
         if(!targe)
         {
             e.error("is expressions within type functions may 
only use type expressions");
             result = ErrorExp.get();
             return ;
         }
         auto targ = ctfeInterpret(targe.exp);
         auto te = targ.isTypeExp();

         result = IntegerExp.createBool(te && te.type && 
te.type.ty != Terror);

         auto tspece = e.tspec ? e.tspec.isTypeExpression() : null;
         auto tspec = tspece ? ctfeInterpret(tspece.exp) : null;
         auto ts = tspec ? tspec.isTypeExp() : null;

         // handling of == and &&
         // See IsExp handling in expressionsem.d
         if (e.tspec && !e.id && !(e.parameters && 
e.parameters.dim))
         {
             if (e.tok == TOK.colon)
             {
                 result = 
IntegerExp.createBool(te.type.implicitConvTo(ts.type) != 
MATCH.nomatch);
             }
             else if(e.tok == TOK.equal)
             {
                 result = 
IntegerExp.createBool(te.type.equals(ts.type));
             }
         }
         else if (e.tok2 != TOK.reserved)
         {
             e.error("Complex pattern matching forms of is 
expressions are not supported in TypeFunctions yet");
             result = IntegerExp.createBool(false);
         }
     }