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This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 119a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-12-20

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 declarationT D whereD has theform

D1 [constant-expression_opt]attribute-specifier-seq_opt

and the type of the identifier in the declarationTD1 is“derived-declarator-type-listT”,then the type of the identifier ofD is an arraytype; if the type of the identifier ofD containstheautotype-specifier, the program isill-formed.

This formulation forbids useful constructs like

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

(accepted by current implementations) and should berelaxed 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 asbeing "not a defect" in consideringissue 1222. However, the use ofauto hassignificantly expanded since that time and the prohibitionof such declarations now seems inconsistent.

Proposed resolution, May, 2021:

1. Change 9.3.4.5 [dcl.array] paragraph 4 asfollows:

U is called the array element type; this type shallnot bea placeholder type(9.2.9.7 [dcl.spec.auto]), a reference type, afunction type, an array of unknown bound,orcvvoid.

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

The return type shall be a non-array object type, areference type, orcvvoid.[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 typeTcontaining a placeholder type, and a correspondinginitializerE, are determined as follows:

• for a non-discardedreturn statementthat occurs in a function declared with a return type thatcontains a placeholder type,T is the declaredreturn type andE is the operand ofthereturn statement. If thereturnstatement has no operand, thenEisvoid();

• for a variable declared with a type that contains aplaceholder type,T is the declared type of thevariable andE is the initializer. If theinitialization is direct-list-initialization, theinitializer shall be abraced-init-list containingonly a singleassignment-expression andE istheassignment-expression;

• for a non-type template parameter declared with atype that contains a placeholder type,T is thedeclared type of the non-type template parameterandE is the corresponding templateargument.

T shall not be an array type. In thecase of areturn statement with no operand...