CWG3045 [basic.scope.block] Expansion statements - redeclaration of for-range-declaration in the body

[jakubjelinek]

Full name of submitter (unless configured in github; will be published with the issue): Jakub Jelinek

Reference (section label): [basic.scope.block]

Link to reflector thread (if any):

Issue description:
P1306R5 added expansion statement next to iteration statement in https://eel.is/c++draft/basic.scope.block#1.1 but has not done the same in https://eel.is/c++draft/basic.scope.block#2.2
I think it would be really surprising if

template for (auto g : whatever) {
  int g = 42;
}

was valid while

for (auto g : whatever) {
  int g = 42;
}

is not.

Suggested resolution:
Change
"substatement of a selection or iteration statement that is not itself a selection or iteration statement, or"
to
"substatement of a selection, iteration or expansion statement that is not itself a selection, iteration or expansion statement, or"

[jakubjelinek]

Also, shouldn't expansion statement be mentioned in [dcl.attr.fallthrough] ?
I mean if expansion stmt has N > 1, then using [fallthrough]]; at the end of the body is very likely an error as the next stmt will not be a labeled statement with case/default label (unless it is conditional on the last iteration). But for N == 1 it easily can. And the N == 0 case is questionable because the body is not instantiated.

switch (x)
  {
  case 1:
     template for (auto i : { 1 })
        {
           foo ();
           [[fallthrough]];  // Is this valid or not?  Next statement has case 2 label, but fallthrough is in the body of expansion statement.
        }
  case 2:
    template for (auto i : {})
       {
          foo ();
          [[fallthrough]]; // What about this?  This won't be instantiated at all.
       }
  case 3:
     template for (auto i : { 1, 2L })
        {
           foo ();
           if (i == 2L)
              [[fallthrough]]; // What about this?  Next statement has case 4 label, but again in expansion statement body.
        }
  case 4:
     bar ();
     break;
  }

[jensmaurer]

CWG3045