A defaulted `operator==` or `operator<=>` compares every base class
subobject, so children of `Printable` couldn't default their
comparisons: `Printable` had no comparison operators of its own, which
made the defaulted operator implicitly deleted. Children that want
member-wise comparison had to write it out by hand instead.
`Printable` is empty, so it now provides comparisons that always compare
equal. Its operands are constrained template parameter rather than
`const Printable&` so that they're only viable for comparing the base
class subobjects themselves. An overload taking `const Printable&` would
also be viable when comparing two `DerivedT` objects by converting them
to the base class, and would then both make children that provide no
comparison silently compare equal and displace the comparisons of
children that provide them through a conversion of their own, as
`EnumBase` does.
Some hand-written comparisons stay, for reasons unrelated to
`Printable`: using `= default` would change their meaning.
Adds `common/ostream_test.cpp`, which covers both the member and friend
forms of defaulting, the resulting comparison categories, and both of
the hazards above.
Assisted-by: Claude Code
---------
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
