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

2611. Missing parentheses in expansion of fold-expression could cause syntactic reinterpretation

Section: 13.7.4  [temp.variadic]     Status: C++23     Submitter: Richard Smith     Date: 2022-08-05

[Accepted as a DR at the November, 2022 meeting.]

13.7.4 [temp.variadic] paragraph 10 expands a fold-expression (including its enclosing parentheses) to an unparenthesized expression. If interpreted literally, this could result in reassociation and misinterpretation of the expression. For example, given:

template<int ...N> int k = 2 * (... + N);

... k<1, 2, 3> is specified as expanding to int k<1, 2, 3> = 2 * 1 + (2 + 3); resulting in a value of 7 rather than the intended value of 12.

Further, there is implementation divergence for the following example:

#include <type_traits>
template<class ...TT>
void f(TT ...tt) {
  static_assert(std::is_same_v<decltype((tt, ...)), int&>);
}
template void f(int /*,int*/);

gcc and MSVC apply the general expression interpretation of decltype, whereas clang and icc apply the identifier special case.

Proposed resolution (approved by CWG 2022-08-26):

Change in 13.7.4 [temp.variadic] paragraph 10 as follows:
The instantiation of a fold-expression (7.5.6 [expr.prim.fold]) produces: ...