This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

2397. auto specifier for pointers and references to arrays

[Accepted as a DR at the June, 2021 meeting.]

According to 9.3.4.5 [dcl.array] paragraph 1,

In a declaration T D where D has the form

D1 [ constant-expression_opt ] attribute-specifier-seq_opt

and the type of the identifier in the declaration T D1 is “derived-declarator-type-list T”, then the type of the identifier of D is an array type; if the type of the identifier of D contains the auto type-specifier, the program is ill-formed.

This formulation forbids useful constructs like

  int a[3];
  auto (*p)[3] = &a;

(accepted by current implementations) and should be relaxed to accommodate such cases.

Notes from the February, 2019 meeting:

CWG agreed that the example should be accepted.

Notes from the May 25, 2021 teleconference:

It was observed that CWG rejected the same example as being "not a defect" in considering issue 1222. However, the use of auto has significantly expanded since that time and the prohibition of such declarations now seems inconsistent.

Proposed resolution, May, 2021:

1. Change 9.3.4.5 [dcl.array] paragraph 4 as follows:

U is called the array element type; this type shall not be a placeholder type (9.2.9.7 [dcl.spec.auto]), a reference type, a function type, an array of unknown bound, or cv void.

2. Change 9.3.4.6 [dcl.fct] paragraph 11 as follows:

The return type shall be a non-array object type, a reference type, or cv void. [Note: An array of placeholder type is considered an array type. —end note]

3. Change 9.2.9.7.2 [dcl.type.auto.deduct] paragraph 2 as follows:

A type T containing a placeholder type, and a corresponding initializer E, are determined as follows:

• for a non-discarded return statement that occurs in a function declared with a return type that contains a placeholder type, T is the declared return type and E is the operand of the return statement. If the return statement has no operand, then E is void();

• for a variable declared with a type that contains a placeholder type, T is the declared type of the variable and E is the initializer. If the initialization is direct-list-initialization, the initializer shall be a braced-init-list containing only a single assignment-expression and E is the assignment-expression;

• for a non-type template parameter declared with a type that contains a placeholder type, T is the declared type of the non-type template parameter and E is the corresponding template argument.

T shall not be an array type. In the case of a return statement with no operand...