From Jason Turner

[stmt.expand]

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tmp/tmpvdqvda1a/{from.md → to.md} RENAMED
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+ ## Expansion statements <a id="stmt.expand">[[stmt.expand]]</a>
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+
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+ Expansion statements specify repeated instantiations
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+ [[temp.decls.general]] of their substatement.
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+
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+ ``` bnf
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+ expansion-statement:
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+ template for '('
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+ init-statementₒₚₜ for-range-declaration ':'
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+ expansion-initializer ')' compound-statement
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+ ```
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+
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+ ``` bnf
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+ expansion-initializer:
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+ expression
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+ expansion-init-list
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+ ```
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+
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+ ``` bnf
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+ expansion-init-list:
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+ '{' expression-listₒₚₜ '}'
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+ ```
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+
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+ The *compound-statement* of an *expansion-statement* is a
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+ control-flow-limited statement [[stmt.label]].
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+
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+ For an expression `E`, let the expressions *`begin-expr`* and
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+ *`end-expr`* be determined as specified in  [[stmt.ranged]]. An
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+ expression is *expansion-iterable* if it does not have array type and
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+ either
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+
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+ - *`begin-expr`* and *`end-expr`* are of the form `E.begin()` and
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+ `E.end()`, or
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+ - argument-dependent lookups for `begin(E)` and for `end(E)` each find
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+ at least one function or function template.
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+
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+ An expansion statement is
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+
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+ - an *enumerating expansion statement* if its *expansion-initializer* is
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+ of the form *expansion-init-list*;
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+ - otherwise, an *iterating expansion statement* if its
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+ *expansion-initializer* is an expansion-iterable expression;
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+ - otherwise, a *destructuring expansion statement*.
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+
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+ An expansion statement S is equivalent to a *compound-statement*
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+ containing instantiations of the *for-range-declaration* (including its
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+ implied initialization), together with the compound-statement of S, as
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+ follows:
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+
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+ - If S is an enumerating expansion statement, S is equivalent to:
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+ ``` cpp
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+ {
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+ init-statement
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+ S₀
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+ ⋮
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+ S_N-1}
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+ }
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+ ```
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+
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+ where N is the number of elements in the *expression-list*, Sᵢ is
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+ ``` cpp
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+ {
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+ for-range-declaration = Eᵢ;
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+ compound-statement
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+ }
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+ ```
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+
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+ and Eᵢ is the iᵗʰ element of the *expression-list*.
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+ - Otherwise, if S is an iterating expansion statement, S is equivalent
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+ to:
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+ ``` cpp
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+ {
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+ init-statement
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+ static constexpr auto&& range = expansion-initializer;
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+ static constexpr auto begin = begin-expr; // see [stmt.ranged]
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+ static constexpr auto end = end-expr; // see [stmt.ranged]
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+
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+ S₀
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+ ⋮
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+ S_N-1}
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+ }
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+ ```
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+
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+ where N is the result of evaluating the expression
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+ ``` cpp
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+ []consteval {
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+ std::ptrdiff_t result = 0;
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+ for (auto i = begin; i != end; ++i, ++result);
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+ return result; // distance from begin to end
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+ }()
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+ ```
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+
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+ and Sᵢ is
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+ ``` cpp
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+ {
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+ static constexpr auto iter = begin + i;
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+ for-range-declaration = *iter;
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+ compound-statement
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+ }
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+ ```
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+
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+ The variables *`range`*, *`begin`*, *`end`*, and *`iter`* are defined
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+ for exposition only.
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+ \[*Note 1*: The instantiation is ill-formed if *`range`* is not a
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+ constant expression [[expr.const]]. — *end note*]
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+ - Otherwise, S is a destructuring expansion statement and S is
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+ equivalent to:
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+ ``` cpp
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+ {
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+ init-statement
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+ \opt{constexpr} auto&& [u₀, u₁, …, u_N-1}] = expansion-initializer;
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+ S₀
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+ ⋮
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+ S_N-1}
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+ }
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+ ```
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+
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+ where N is the structured binding size of the type of the
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+ *expansion-initializer* and Sᵢ is
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+ ``` cpp
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+ {
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+ for-range-declaration = uᵢ;
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+ compound-statement
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+ }
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+ ```
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+
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+ The keyword `constexpr` is present in the declaration of
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+ $u_{0}, u_{1}, \dotsc, u_{N-1}$ if and only if `constexpr` is one of
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+ the *decl-specifier*s of the *decl-specifier-seq* of the
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+ *for-range-declaration*.
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+
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+ [*Example 1*:
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+
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+ ``` cpp
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+ consteval int f(auto const&... Containers) {
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+ int result = 0;
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+ template for (auto const& c : {Containers...}) { // OK, enumerating expansion statement
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+ result += c[0];
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+ }
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+ return result;
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+ }
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+ constexpr int c1[] = {1, 2, 3};
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+ constexpr int c2[] = {4, 3, 2, 1};
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+ static_assert(f(c1, c2) == 5);
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+ ```
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+
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+ — *end example*]
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+
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+ [*Example 2*:
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+
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+ ``` cpp
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+ consteval int f() {
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+ constexpr std::array<int, 3> arr {1, 2, 3};
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+ int result = 0;
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+ template for (constexpr int s : arr) { // OK, iterating expansion statement
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+ result += sizeof(char[s]);
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+ }
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+ return result;
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+ }
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+ static_assert(f() == 6);
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+ ```
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+
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+ — *end example*]
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+
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+ [*Example 3*:
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+
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+ ``` cpp
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+ struct S {
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+ int i;
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+ short s;
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+ };
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+
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+ consteval long f(S s) {
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+ long result = 0;
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+ template for (auto x : s) { // OK, destructuring expansion statement
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+ result += sizeof(x);
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+ }
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+ return result;
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+ }
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+ static_assert(f(S{}) == sizeof(int) + sizeof(short));
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+ ```
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+
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+ — *end example*]
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+