before this change, we assumed that the dereference types of
the given `InputIterator1` and `InputIterator2` are always
references. but this does not hold if the `operator*` returns
a rvalue, as in the C++20 standard, unlike the LegacyForwardIterator
requirement, `std::forward_iterator` does not requires
dereference to return a reference. so we should not assume this,
if we want to use `combine()` with iterators whose dereference
return a, for instance, rvalue.
in this change, we use `std::iter_const_reference_t` instead. this
type is deduced from the behavior of the iterator instead of hardwire
it to a reference type. this allows us to use a C++20 forward_iterator
with this generic function.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Instead of lengthy blurbs, switch to single-line, machine-readable
standardized (https://spdx.dev) license identifiers. The Linux kernel
switched long ago, so there is strong precedent.
Three cases are handled: AGPL-only, Apache-only, and dual licensed.
For the latter case, I chose (AGPL-3.0-or-later and Apache-2.0),
reasoning that our changes are extensive enough to apply our license.
The changes we applied mechanically with a script, except to
licenses/README.md.
Closes#9937
