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### 3615. The last specialization of `incrementable_traits` has wrong operand types

Section: 25.3.2.1 [incrementable.traits] Status: New Submitter: Hewill Kang Opened: 2021-09-30 Last modified: 2022-01-31

Priority: 3

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Discussion:

The last specialization of `incrementable_traits` requires `a - b` to be well-formed, where the types of both operands are `const` lvalue reference of type `T`. However inside the `struct`, it uses `decltype(declval<T>() - declval<T>())` to define the `difference_type`, that is, non-`const` rvalue reference of type `T`.

[2022-01-29; Reflector poll]

Set priority to 3 after reflector poll that failed to reach consensus. Some suggested NAD: "Implicit expression variations ([concepts.equality]/6) apply here."

Proposed resolution:

This wording is relative to N4892.

1. Modify 25.3.2.1 [incrementable.traits] as indicated:

   >     namespace std {
   >       […]
   >       template<class T>
   >         requires (!requires { typename T::difference_type; } &&
   >                   requires(const T& a, const T& b) { { a - b } -> integral; })
   >       struct incrementable_traits<T> {
   >         using difference_type = make_signed_t<decltype(declval<const T&>() - declval<const T&>())>;
   >       };
   >       […]
   >     }