At least there's a dmd bug.

For such static array assignment, dmd should use druntime function, but instead plain memory copy operation used.

https://github.com/D-Programming-Language/dmd/blob/master/src/e2ir.c#L2920

https://github.com/D-Programming-Language/dmd/blob/master/src/e2ir.c#L2945

And, even after the bug is fixed, I think it's debatable behavior whether an element-wise assignment should call opAssign on each elements, because it can be a kind of optimization.