[stmt.while]

When the condition of a while statement is a declaration, the scope of the variable that is declared extends from its point of declaration ([basic.scope.pdecl]) to the end of the while statement.

A while statement of the form

while (T t = x) statement

is equivalent to

label:
{                   // start of condition scope
  T t = x;
  if (t) {
    statement
    goto label;
  }
}                   // end of condition scope

The variable created in a condition is destroyed and created with each iteration of the loop.

[ Example

struct A {
  int val;
  A(int i) : val(i) { }
  ~A() { }
  operator bool() { return val != 0; }
};
int i = 1;
while (A a = i) {
  // ...
  i = 0;
}

In the while-loop, the constructor and destructor are each called twice, once for the condition that succeeds and once for the condition that fails.

— end example

]

9 Statements [stmt.stmt]

9.5 Iteration statements [stmt.iter]

9.5.1 The while statement [stmt.while]