[Issue 19441] alias this causes partial assignment

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19441

--- Comment #6 from Eyal <eyal at weka.io> ---
Stefan - under what circumstances does prioritizing "alias this" ever help?

Anyway, a compiler error - not changed semantics - as proposed here, should be
fine (with the ordinary caveat of __traits(compiles ...)).

Also, currently it may inconsistently choose to assign the outer or inner field
in different circumstances, which is clearly the worst of all worlds.

Assignment *always* assigning the whole lvalue is easier to explain and far
less error-prone.

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