[Issue 21235] Undefined reference with circular opEquals between two templates, one of which has an error

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21235

--- Comment #2 from FeepingCreature <default_357-line at yahoo.de> ---
This may be a duplicate of https://issues.dlang.org/show_bug.cgi?id=19091 . It
smells similar, but I'm not certain. The `opEquals` seems to add a unique touch
here, but they may have a common cause.

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