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

Question

I can't find any information about new C++17 if initializer syntax and 'constexpr if' in:

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

Nevertheless, the syntax is supported by Clang-HEAD...

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

online code here -> http://melpon.org/wandbox/permlink/dj3a9ChvjhlNc8nr

Is the constexpr if with initializer guaranteed by the standard, as constexpr if is just an "if with constexpr" or it's not guaranteed and has to be explicitly added to the standard?

Answer 1

The Selection statements with initializer proposal mentions if constexpr, and states "the facilities of if constexpr work just as well with the extended if statement from this proposal".

The specification about if statement with initializer in N4606 [stmt.if]p3 explicitly allows use of if constexpr.

Here is what N4606 [stmt.if]p3 says:

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.