[Issue 16657] alias this interacts with generated opCmp and opEquals

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=16657

--- Comment #10 from Eyal <eyal at weka.io> ---
I think a new "transition" compiler flag [1] can be added to warn about the
changes in behavior here.

It is possible to make these warnings the default for a few versions (with a
flag to suppress them), but I think code relying on this bug for correctness is
probably very rare. One can always opt-in to all transition warnings via
-transition=all when upgrading the compiler.

This seems in line with how D warns about other language changes.

[1] https://dlang.org/dmd-linux.html#switch-transition

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