On 8/9/17 2:25 PM, Nordlöw wrote:
Why doesn't appending to `subs` work with std.array.Appender in

     struct T
     {
         string src;
         import std.array : Appender;
         Appender!(T[]) subs;
     }
     T t;
     t.subs ~= T.init; // ERRORS
     t.subs.put(T.init); // ERRORS

when it works with builtin arrays as in

     struct S
     {
         string src;
         S[] subs;
     }
     S s;
     s.subs ~= S.init;

?

Specifically

     t.subs ~= T.init

errors as

     Error: cannot append type T to type Appender!(T[])

Here is the problem:

ElementType!(T[]) is void.

Here is ElementType:
template ElementType(R)
{
    static if (is(typeof(R.init.front.init) T))
        alias ElementType = T;
    else
        alias ElementType = void;
}

So what is happening here (I think), is that T isn't fully defined, so T.init.front.init is an error at this point. Therefore the else clause is selected.

This is one of the problems with using is(typeof) (or __traits(compiles)) with an "else" clause. You may not be checking what you think you are checking, and end up with the wrong result.

So essentially, Appender!(T[]) inside a T becomes (essentially) Appender!(void[]). And you can't append a T to a void[].

If I change the definition of ElementType to use R.init.front instead of R.init.front.init, it compiles. But I'm pretty sure this will break other ranges.

Somewhere, there's an answer.

-Steve

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