constexpr if with initializer 由标准保证吗？ 'constexpr(constexpr auto x = f(); x) { }'

Question

我找不到任何关于新 C++17 if 初始化语法的信息和 'constexpr if' 在：

http://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0128r1.html

尽管如此，Clang-HEAD...

支持该语法

constexpr auto f() { return true; }
int main() {
    if constexpr(constexpr auto x = f(); x) { }
}

在线代码在这里 -> http://melpon.org/wandbox/permlink/dj3a9ChvjhlNc8nr

constexpr if with initializer 是否由标准保证，因为 constexpr if 只是一个“if with constexpr”，或者它没有保证并且必须明确添加到标准？

Answer 1

Selection statements with initializer 提案提到 if constexpr，并声明 "the facilities of if constexpr work just as well with the extended if statement from this proposal"。

N4606 [stmt.if]p3 中关于带有初始值设定项的 if 语句的规范明确允许使用 if constexpr.

这是 N4606 [stmt.if]p3 所说的：

An if statement of the form
if constexpr[opt] ( init-statement condition ) statement
is equivalent to
{
  init-statement
  if constexpr[opt] ( condition ) statement
}
and an if statement of the form
if constexpr[opt] ( init-statement condition ) statement else statement
is equivalent to
{
  init-statement
  if constexpr[opt] ( condition ) statement else statement
}
except that names declared in the init-statement are in the same declarative region as those declared in the condition.

constexpr if with initializer 由标准保证吗？ 'constexpr(constexpr auto x = f(); x) { }'

Is constexpr if with initializer guaranteed by the standard? 'constexpr(constexpr auto x = f(); x) { }'

c++

clang

constexpr

c++17