P2997 - Removing the common reference requirement from the indirectly invocable concepts

This paper is making iter_common_reference_t unused in the standard library (except for range_common_reference_t which is unsed now). Implementations (libc++, libstdc++, and MSVC STL) currently don't use iter_common_reference_t in product code other than specified by the standard.

(iter_common_reference_t and range_common_reference_t are seldomly used in some test codes.)

Per search on Github, it seems that almost no one (other than implentations of ranges) uses iter_common_reference_t.

I've only found such things explaining the motivation of iter_common_reference_t/range_common_reference_t (authored by @ericniebler).

Details

[P0022R2](https://wg21.link/p0022r2) > If using a polymorphic lambda is undesirable, an alternate solution is to use the iterator’s common reference type: > > ```C++ > // Use the iterator's common reference type to define a monomorphic relation: > using X = iter_common_reference_t; > sort(first, last, [](X&& x, X&& y) {return x < y;}); > ``` [Answer on stackoverflow](https://stackoverflow.com/a/59024241) > EDIT: The OP also asked for an example. This is a little contrived, but imagine it's C++20 and you are given a random-access range `r` of type `R` about which you know nothing, and you want to sort the range. > > Further imagine that for some reason, you want to use a monomorphic comparison function, like `std::less`. (Maybe you've type-erased the range, and you need to also type-erase the comparison function and pass it through a `virtual`? Again, a stretch.) What should `T` be in `std::less`? For that you would use `common_reference`, or the helper `iter_common_reference_t` which is implemented in terms of it. > > ```C++ > using CR = std::iter_common_reference_t>; > std::ranges::sort(r, std::less{}); > ``` > > That is guaranteed to work, even if range `r` has proxy iterators. And the paper P2997 itself.

But the desired pattern doesn't seem actually used (per search on Github).

Would it be better to write a new paper to deprecate/remove iter_common_reference_t?

brevzin / cpp_proposals

P2997 - Removing the common reference requirement from the indirectly invocable concepts #130