[Issue 14145] opDispatch not considered when function body doesn't compile

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=14145

--- Comment #10 from hsteoh at quickfur.ath.cx ---
I've also run into this before, and it's nigh impossible to debug if you didn't
realize opDispatch was involved, because declining instantiation rather than
generating a compile error may cause overload resolution to move to another,
unintended overload, which may in turn have some deeply-nested templates which
fails in some deeply-nested scope. You'd trace the error all the way back out
the entire instantiation chain, then be left scratching your head as to why
that template was even chosen in the first place, and it may take a while to
realize opDispatch was the intended call target but due to suppression of
errors it was silently skipped.

As Steven said, whether opDispatch is chosen should not depend on the body, but
only on the declaration and sig constraints. Any errors in the body should be a
hard compile error.  For method-forwarding templates, it should be easy enough
to use __traits(hasMember) in opDispatch's sig constraints to decide whether or
not to accept instantiation. Small price for greatly improving error reporting
involving opDispatch.

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