iex(14)> Map.to_list(%YY{})                
[__struct__: YY, field: nil]
iex(15)> Map.to_list(YY.__struct__())
[__struct__: XX, field: nil]

Kernel.SpecialForms.%/2

Based on the evidence I suspect that the compiled code (it’s a special form after all) sanitizes the value returned from the function by forcing the __struct__ value to reflect the primary module - but that is just a guess.

This issue has been solved here.

iex(16)> struct!(YY,field: 1)
%XX{field: 1}
iex(17)> struct!(%YY{},field: 1)
%YY{field: 1}

I suspect the bug is with Kernel.struct!/2, not Kernel.SpecialForms.%/2.

As you discovered, it only presents when you start defdelegating the constructor and I don’t think this code anticipated that.

So the question is - what are you trying to accomplish by defdelegating the struct constructor to another struct? Sharing code between XX and YY should probably be accomplished via a shared third module.