[Bug libstdc++/107850] [12 Regression] std::erase_if (map) forces predicate to takes a const value_type

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107850

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Charles-Henri Gros from comment #4)
> Looking into it further, there may be an implicit requirement that the
> predicate does not modify its argument.
> https://eel.is/c++draft/algorithms.requirements#6

See paragraph 4. That only applies to the Algorithms clause, not the Containers
clause. So it doesn't apply to std::erase_if for maps. It *does* apply to
std::erase_if for sequence containers, because those are defined in terms of
std::remove_if, which is defined in the Algorithms clause. It doesn't apply to
std::erase_if for sets, but *first is always const for those anyway so it's
irrelevant.

> I'm unfortunately not well-versed enough in C++ legalese to tell what
> "possibly const" means in that context,

A "glvalue u of type (possibly const) T" means u is a T& or const T&. Your
predicate would fail that requirement (if it applied here) because it cannot be
called with const T& at all, so pred(u) isn't a valid expression.

> nor "apply any non-constant function".

It means you can't modify the argument.

> And while I understand that a "predicate" is generally meant to not do
> modification, there are fairly frequent use cases for "apply this
> potentially modifying operation, and depending on its result remove the
> element from the container".

You can write your own "modify and remove if" algo for that.

> And in practice, this works (e.g.
> std::remove_if, your own earlier version of erase_if, other compilers'
> version...).

It's undefined behaviour to do that with std::remove_if.

> Maybe I should go to LEWG and lobby to remove that limitation. In the
> meanwhile, I'll probably just keep my own, perhaps slightly less optimal
> removal algorithms.

There is no such restriction on std::erase_if for maps today, but I'll be
lobbying the committee to *add* it!