This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 110c. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2023-02-07


648. Constant expressions in constexpr initializers

Section: 9.2.6  [dcl.constexpr]     Status: CD1     Submitter: Mike Miller     Date: 12 Aug 2007

[Voted into the WP at the September, 2008 meeting.]

The current wording of 9.2.6 [dcl.constexpr] paragraph 7 seems not quite correct. It reads,

A constexpr specifier used in an object declaration declares the object as const. Such an object shall be initialized, and every expression that appears in its initializer (9.4 [dcl.init]) shall be a constant expression.

The phrase “every expression” is intended to cover multiple arguments to a constexpr constructor and multiple expressions in an aggregate initializer. However, it could be read (incorrectly) as saying that non-constant expressions cannot appear as subexpressions in such initializers, even in places where they do not render the full-expression non-constant (i.e., as unevaluated operands and in the unselected branches of &&, ||, and ?:). Perhaps this problem could be remedied by replacing “every expression” with “every full-expression?”

Proposed resolution (June, 2008):

Change 9.2.6 [dcl.constexpr] paragraph 7 as follows:

A constexpr specifier used in an object declaration declares the object as const. Such an object shall be initialized, and every expression that appears in its initializer (8.5) initialized. If it is initialized by a constructor call, the constructor shall be a constexpr constructor and every argument to the constructor shall be a constant expression. Otherwise, every full-expression that appears in its initializer shall be a constant expression. Every implicit conversion used...