Can't use is expression on nested type

[Ali Çehreli]

On 3/13/25 5:14 PM, Emma wrote:

 > but the below fails with a “undefined identifier 'S'” error!
[...]
 > bool foo(T)(T it) {
 >      static if (is(T == X.Foo!S, string S)) {

Here is another solution that uses std.traits:

import std.traits;

struct X {
     struct Foo(string S) {}
}

bool foo(T)(T it) {
     static if (isInstanceOf!(T, X.Foo)) {
         alias S = templateArgsOf!T[0];

         // The following passes:
         // static assert(is (S == string));

         return true;

     } else {
         return false;
     }
}

void main() {
     foo(X.Foo!"x"());
}

Ali