An example of weird template error messaging

[ag0aep6g]

On 04.08.19 17:18, Ethan wrote:
> ----------
> 
> template ApplyRight( alias Template, Right... )
> {
>      alias ApplyRight( Left... ) = Template!( Left, Right );
> }
> 
> struct ObjectRefImpl( Type, BaseType ) if( is( Type Super == super ) && 
> is( Super == BaseType ) )
> {
>      Type refval;
> }
> 
> alias ObjRef = ApplyRight!( ObjectRefImpl, ObjectBase );
> 
> class ObjectBase
> {
>      string name;
> }
> 
> alias ObjectOne = ObjRef!ObjectOneImpl;
> 
> class ObjectOneImpl : ObjectBase
> {
>      ObjectTwo two;
> }
> 
> alias ObjectTwo = ObjRef!ObjectTwoImpl;
> 
> class ObjectTwoImpl : ObjectBase
> {
>      ObjectOne one;
> }
> 
> void main()
> {
>      import std.stdio : writeln;
> 
>      ObjectOne one = { new ObjectOneImpl() };
>      writeln( "Oh hi Mark" );
> }
> 

Reduced further:

----
struct ObjectRefImpl1() if (is(ObjectOneImpl Super == super)) {}
struct ObjectRefImpl2() if (is(ObjectTwoImpl Super == super)) {}

alias ObjectOne = ObjectRefImpl1!();

class ObjectOneImpl
{
     alias Two = ObjectRefImpl2!();
}

class ObjectTwoImpl
{
     ObjectOne one; /* Error: template instance `test.ObjectRefImpl1!()` 
is used as a type */
}
----

Compilation succeeds if `ObjectOne` is moved below `ObjectOneImpl`.