Checking for manifest constants

[Basile B.]

On Friday, 5 March 2021 at 08:23:09 UTC, Bogdan wrote:
> I was using a trick with dmd to check for manifest constants 
> which worked until dmd v2.094. Yesterday I tried it on the 
> latest compiler and it failed with:
>
>
> source/introspection/manifestConstant.d(37,28): Error: need 
> this for name of type string
> source/introspection/type.d(156,13): Error: value of this is 
> not known at compile time
>
> any ideas how to fix it? or, is it a bug with dmd?
>
> ```
>
> /// Check if a member is manifest constant
> bool isManifestConstant(T, string name)() {
>   mixin(`return is(typeof(T.init.` ~ name ~ `)) && 
> !is(typeof(&T.init.` ~ name ~ `));`);
> }
>
> /// ditto
> bool isManifestConstant(alias T)() {
>   return is(typeof(T)) && !is(typeof(&T));
> }
>
> enum globalConfig = 32;
> int globalValue = 22;
>
> unittest {
>   struct Test {
>     enum config = 3;
>     int value = 2;
>   }
>
>   static assert(isManifestConstant!(Test.config));
>   static assert(isManifestConstant!(Test, "config"));
>   static assert(isManifestConstant!(globalConfig));
>
>   static assert(!isManifestConstant!(Test.value));
>   static assert(!isManifestConstant!(Test, "value"));
>   static assert(!isManifestConstant!(globalValue));
> }
>
> void main() {}
>
>
> ```

Hello, you can use this template instead:

   template isManifestConstant(alias V, T...)
   if (T.length == 0 || (T.length == 1 && is(T[0])))
   {
     enum isKnown = is(typeof((){enum v = V;}));
     static if (!T.length)
         enum isManifestConstant = isKnown;
     else
         enum isManifestConstant = isKnown && is(typeof(V) == 
T[0]);
   }

The optional T is to verify if it is a compile time constant of a 
certain type.
the tests you wrote and that are not based on a string pass.