Deduction guide for std::array

Abstract

[US 148] suggests to add the following deduction guide to std::array

template <class T>
array(T&&...) -> array<common_type_t<T...>, sizeof...(T)>;

, which is similar to make_array‘s behavior as presented in N4600 Library Fundamentals V2. This paper suggests that rather than deducing to a “common type”, same-decayed type should be required. Meaning,

array{ 1, 2u }

will be banned, and

array{ "literal", ref_char_ptr }

will be preserved.

Discussion

The author of the make_array proposal is aware of several drawbacks with the “common type” behavior and put efforts in the adopted facility to address them. For example, an array of common type can be fragile. The deduced “common type” is often unintended, and a later added element can change the type of the entire array, and this change may not be captured at compile-time. When a user runs into these issues, he/she can switch to make_array<T>, which simply specifies the element type without deduction, but this option is not available to the deduction guide for array since we decided not to deduce a template-id.

The trait common_type_t can also be fragile. A typical example is that, given the deduction guide specified in [US 148] unmodified,

array{ ref(lv_int), ref(lv_int) }

gives array<reference_wrapper<int>> which is not bad, but

array{ ref(lv_int), ref(lv_long) }

will give you an array<long> that is not even mentioning reference_wrapper, and that’s one of the major reasons why make_array banned reference_wrapper. Given the discussion happened in LEWG, we are not sure whether we want to unwrap references with tuple and pair‘s deduction guides and I’m not sure whether I want to do anything special to array’s deduction guide in this area.

The “common type” semantics does not present in other tuple-like types. To rephrase this in an unfortunately academic way, the other two product types, namely tuple and pair, are not trying to promote type arguments to other types with larger ranges and end up with creating types that are no longer product types. A product type such as tuple<X, Y> is called product type because the possible range of values, denoted as |tuple<X,Y>|, is |X|×|Y|, not |Wat|².

Last but not least, it is quite obvious that

vector{ 1, 2.0, 3u }

does not give you a vector<double>, so why expect std::array to do so given exactly the same syntax?

Wording

This wording is relative to N4606.

Add the following signature to 23.3.7.1 [array.overview]/3:

Add the following as paragraph 2 to 23.3.7.2 [array.cons]:

Remarks: D is determined as follows: Let U_i be decay_t<T_i> for each T_i in T. Then D is U₀ and is_same_v<D, U₀> is true for all i. If there is no such D, the program is ill-formed.

Implementation

Tested with gcc-7.0.0: Wandbox.