Re: Checking for manifest constants

[Basile B. via Digitalmars-d-learn]

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.