P0202R3 constexpr For <algorithm> And exchange()

[StephanTLavavej]

P0202R3 constexpr For <algorithm> And exchange()

LWG-3256 Feature testing macro for constexpr algorithms

This needs #156 is_constant_evaluated().

Feature-test macro as of WG21-N4842:

• If this feature is implemented, but P0879R0 constexpr For Swapping Functions #38 WG21-P0879 is not implemented:
  #define __cpp_lib_constexpr_algorithms 201703L
• If both this feature and P0879R0 constexpr For Swapping Functions #38 WG21-P0879 are implemented:
  #define __cpp_lib_constexpr_algorithms 201806L

[BillyONeal]

I'm starting this and hope to get the <xutility> family constexprized shortly.

[BillyONeal]

Should we turn on the feature test macro for this even though some algorithms will break due to DevCom-883631 ?

[BillyONeal]

(e.g. so far some calls to is_permutation will trigger it)

[CaseyCarter]

Should we turn on the feature test macro for this even though some algorithms will break due to DevCom-883631 ?

Yes, I believe we should. I wouldn't say that we don't support is_permutation at runtime if there were argument types for which the compiler generates bogus diagnostics or values for which it generates bad code, and this seems like the constexpr flavor of such cases.

[BillyONeal]

p0202r3, those in bold have libc++ tests already present we can use:

• all_of
• any_of
• none_of
• for_each
• for_each_n
• find
• find_if
• find_if_not
• find_end
• find_first_of
• adjacent_find
• count
• count_if
• mismatch
• equal
• is_permutation
• search
• search_n
• copy
• copy_n
• copy_if
• copy_backward
• move
• move_backward
• transform
• replace
• replace_if
• replace_copy
• replace_copy_if
• fill
• fill_n
• generate
• generate_n
• remove
• remove_if
• remove_copy
• remove_copy_if
• unique
• unique_copy
• reverse_copy
• rotate_copy
• is_partitioned
• partition_copy
• partition_point
• is_sorted
• is_sorted_until
• lower_bound
• upper_bound
• equal_range
• binary_search
• merge
• includes
• set_union
• set_intersection
• set_difference
• set_symmetric_difference
• is_heap
• is_heap_until
• lexicographical_compare
• exchange

[BillyONeal]

I asked libc++ folks how they would like submitted tests for this since they would fail, and didn't get a reasonable answer, so I'm going to go ahead and do an internal-only test for the missing algorithms, as well as the ones that needed explicit is_constant_evaluated tests.