This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 114a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-04-18

1407. Integral to bool conversion in converted constant expressions

Section: 7.7  [expr.const]     Status: NAD     Submitter: Richard Smith     Date: 2011-10-21

The following example appears to be ill-formed, although current implementations accept it:

  template<bool> struct S { };
  S<0> s;

The reason this is ill-formed is that the non-type template argument is a converted constant expression of type bool (see 13.4.3 [temp.arg.nontype] paragraph 5), and the second conversion in the implicit conversion sequence is a boolean conversion, which is not allowed in the conversion for a converted constant expression (see 7.7 [expr.const] paragraph 3) . Conversions in the other direction (from bool to integer types) are permitted here, since they're integral promotions.

Rationale (February, 2012):

The analysis is correct, and the example is ill-formed. Implementations that accept it are in error.