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### 2357. Remaining "`Assignable`" requirement

Section: 25.8.5 [alg.partitions] Status: C++14 Submitter: Daniel Krügler Opened: 2014-02-01 Last modified: 2017-07-05

Priority: 0

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Discussion:

The Requires element of `partition_copy` says (emphasis mine):

> Requires: `InputIterator`'s value type shall be `Assignable`, and …

The C++03 term `Assignable` was replaced by `CopyAssignable`, remaining cleanups happened via LWG issue 972, but algorithm `partition_copy` was not affected at that time (during that time the requirements of `partition_copy` didn't mention writable nor assignable, but I cannot track down at the moment where these requirements had been added). Presumably this requirement should be corrected similarly to the approach used in 972.

Another question is whether a `CopyAssignable` is needed here, given the fact that we already require "writable to" an `OutputIterator` which is defined in 23.3.1 [iterator.requirements.general] and does already impose the necessary statement

>     *out = *in;

Given the fact that `partition_copy` never touches any input value twice, there is no reason why anything more than writable to should be necessary.

The below suggested primary resolution does not respond to the second part of this question.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3797.

1. Modify 25.8.5 [alg.partitions], p12 as indicated:

   > -12- Requires: `InputIterator`'s value type shall be `CopyAssignable`, and shall be writable to the `out_true` and `out_false` `OutputIterator`s, and shall be convertible to `Predicate`'s argument type. The input range shall not overlap with either of the output ranges.