[stmt.expand]

The compound-statement of an expansion-statement is a control-flow-limited statement ([stmt.label]).

For an expression E, let the expressions begin-expr and end-expr be determined as specified in [stmt.ranged].

An expression is expansion-iterable if it does not have array type and either

(3.1)
begin-expr and end-expr are of the form E.begin() and E.end(), or
(3.2)
argument-dependent lookups for begin(E) and for end(E) each find at least one function or function template.

An expansion statement is

(4.1)
an enumerating expansion statement if its expansion-initializer is of the form expansion-init-list;
(4.2)
otherwise, an iterating expansion statement if its expansion-initializer is an expansion-iterable expression;
(4.3)
otherwise, a destructuring expansion statement.

An expansion statement S is equivalent to a compound-statement containing instantiations of the for-range-declaration (including its implied initialization), together with the compound-statement of S, as follows:

(5.1)
If S is an enumerating expansion statement, S is equivalent to: { init-statement
(5.2)
Otherwise, if S is an iterating expansion statement, S is equivalent to: { init-statement constexpr auto& range = expansion-initializer; constexpr auto begin = begin-expr; / see [stmt.ranged] constexpr auto end = end-expr; / see [stmt.ranged] $S0 ⋮ SN−1 } where N is the result of evaluating the expression [] consteval { std::ptrdiff_t result = 0; for (auto i = begin; i != end; ++i) ++result; return result; / distance from begin to end }() and Si is { constexpr auto iter = begin + decltype(begin - begin){i}; for-range-declaration = *iter; compound-statement } The variables range, begin, end, and iter are defined for exposition only. The identifier i is considered to be a prvalue of type std::ptrdiff_t; the program is ill-formed if i is not representable as such a value. [Note 1: The instantiation is ill-formed if range is not a constant expression ([expr.const]). — end note]$
(5.3)
Otherwise, S is a destructuring expansion statement and, if N is 0, S is equivalent to: { init-statement constexpr ${o p t auto & range = expansion-initializer;} otherwise, S is equivalent to: {init-statement constexpr {o p t auto & [{u 0, {u 1, \dots, {u N - 1] =;}_{N - 1] =;}}_{1, \dots, {u N - 1] =;}_{N - 1] = expansion-initializer;}}}_{0, {u 1, \dots, {u N - 1] =;}_{N - 1] = expansion-initializer;}}_{1, \dots, {u N - 1] = expansion-initializer;}_{N - 1] = expansion-initializer;}}}}_{o p t auto & [{u 0, {u 1, \dots, {u N - 1] =;}_{N - 1] = expansion-initializer;}}_{1, \dots, {u N - 1] = expansion-initializer;}_{N - 1] = expansion-initializer;}}}_{0, {u 1, \dots, {u N - 1] = expansion-initializer;}_{N - 1] = expansion-initializer;}}_{1, \dots, {u N - 1] = expansion-initializer;}_{N - 1] = expansion-initializer;}}}}}_{o p t auto & range = expansion-initializer;} otherwise, S is equivalent to: {init-statement constexpr {o p t auto & [{u 0, {u 1, \dots, {u N - 1] =;}_{N - 1] = expansion-initializer;}}_{1, \dots, {u N - 1] = expansion-initializer;}_{N - 1] = expansion-initializer;}}}_{0, {u 1, \dots, {u N - 1] = expansion-initializer;}_{N - 1] = expansion-initializer;}}_{1, \dots, {u N - 1] = expansion-initializer;}_{N - 1] = expansion-initializer;}}}}_{o p t auto & [{u 0, {u 1, \dots, {u N - 1] = expansion-initializer;}_{N - 1] = expansion-initializer;}}_{1, \dots, {u N - 1] = expansion-initializer;}_{N - 1] = expansion-initializer;}}}_{0, {u 1, \dots, {u N - 1] = expansion-initializer;}_{N - 1] = expansion-initializer;}}_{1, \dots, {u N - 1] = expansion-initializer;}_{N - 1] = expansion-initializer;}}}}}$

[Example 1: consteval int f(auto const&. Containers) { int result = 0; template for (auto const& c : {Containers..}) { / OK, enumerating expansion statement result += c[0]; } return result; } constexpr int c1[] = {1, 2, 3}; constexpr int c2[] = {4, 3, 2, 1}; static_assert(f(c1, c2) == 5); — end example]

[Example 2: consteval int f() { constexpr std::array<int, 3> arr {1, 2, 3}; int result = 0; template for (constexpr int s : arr) { / OK, iterating expansion statement result += sizeof(char[s]); } return result; } static_assert(f() == 6); — end example]

[Example 3: struct S { int i; short s; }; consteval long f(S s) { long result = 0; template for (auto x : s) { / OK, destructuring expansion statement result += sizeof(x); } return result; } static_assert(f(S{}) == sizeof(int) + sizeof(short); — end example]