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C++ 6.4p1 (selection-statements):
selection-statement:
if ( condition ) statement
if ( condition ) statement else statement
switch ( condition ) statement

C++ 6.4p2:
The rules for conditions apply both to selection-statements and to the for and while statements

C++ 6.4p3:
A name introduced by a declaration in a condition is in scope from its point of declaration until the end of the substatements controlled by the condition. If the name is re-declared in the outermost block of a substatement controlled by the condition, the declaration that re-declares the name is ill-formed.
Example:

Code: Select all

if (int x = f()) {
  int x; // ill-formed, redeclaration of x
}
else {
  int x; // ill-formed, redeclaration of x
}

int f();
void g() {
  if (int x=f()) {
    int x; // error, PASS
  }
  switch (int x=f()) {
    default:
      int x; // no error, FAIL
  }
  while (int x=f()) {
    int x; // error, PASS
  }
  for (;int x=f();) {
    int x; // error, PASS
  }
}

int f();
void g() {
  if (int x=f()) {
    int x; // no error, FAIL
  }
  switch (int x=f()) {
    default:
      int x; // no error, FAIL
  }
  while (int x=f()) {
    int x; // error, PASS
  }
  for (;int x=f();) {
    int x; // error, PASS
  }
}

C++ 6.5.3p1 (The for statement)
The for statement:
for ( for-init-statement condition[optinal] ; expression[optional] ) statement

Names declared in the for-init-statement are in the same declarative-region as those declared in the condition.

int f();
void g() {
  for (int x; int x=f();) {} // redeclaration error, PASS
}

int f();
void g() {
  for (int x; int x=f();) {} // no error, FAIL
}