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### 3297. Useless sequence container requirement

Section: 22.2.3 [sequence.reqmts] Status: New Submitter: Casey Carter Opened: 2019-09-17 Last modified: 2020-09-06

Priority: 3

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Discussion:

22.2.3 [sequence.reqmts] paragraph 3 says that the names `i` and `j` denote "iterators that meet the Cpp17InputIterator requirements and refer to elements implicitly convertible to `value_type`". Ignoring for the moment that this is an occurrence of LWG 3105 — we really mean that `*i` and `*j` must be implicitly convertible to `value_type` — this requirement seems to be completely extraneous.

The names `i` and `j` are used in three places in the requirements table:

- The range constructors `X(i, j)` and `X u(i, j)`, which require that the container's value type is Cpp17EmplaceConstructible into the container from `*i`; implicit conversion is neither necessary nor sufficient.

- The range insert overload `a.insert(p, i, j)` which also requires Cpp17EmplaceConstructible, as well as the capability to move elements around for `vector` and `deque`; again, implicit conversion is neither necessary nor sufficient. It would be useful / performant here to require that the container's value type is assignable from `*i`, which may have been the intent of the implicit conversion requirement — would doing so be too breaking?

- The range assign overload `a.assign(i, j)` which requires both Cpp17EmplaceConstructible as above and that it can assign the result of dereferencing an iterator directly to the container's value type; again, implicit conversion is not useful here.

We should strike the implicit conversion requirement since it is not useful and only serves to confuse readers of the Standard (see e.g. here).

[2019-10-31 Issue Prioritization]

Priority to 3 after reflector discussion.

Proposed resolution:

This wording is relative to N4830.

1. Modify 22.2.3 [sequence.reqmts] as indicated:

   > -3- In Tables 76 and 77, `X` denotes a sequence container class, `a` denotes a value of type `X` containing elements of type `T`, `u` denotes the name of a variable being declared, `A` denotes `X::allocator_type` if the qualified-id `X::allocator_type` is valid and denotes a type (13.10.3 [temp.deduct]) and `allocator<T>` if it doesn't, `i` and `j` denote iterators that meet the Cpp17InputIterator requirements and refer to elements implicitly convertible to `value_type`, `[i, j)` denotes a valid range, […]