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

The instantiation of a fold-expression (7.5.7 [expr.prim.fold]) produces:

• ( ((E₁ op E₂ ) op . . . ) op E_N ) for a unary left fold,
• ( E₁ op (. . . op (E_N-1 op E_N )) ) for a unary right fold,
• ( (((E op E₁ ) op E₂ ) op . . . ) op E_N ) for a binary left fold, and
• ( E₁ op (. . . op (E_N-1 op (E_N op E))) ) for a binary right fold.

...

[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.