[Voted into WP at August, 2010 meeting.]

Note to editor:

The formatting in the preceding change for range-init follows that of the existing text for begin-expr and end-expr. However, CWG is concerned that this style makes all of these elements look too much like grammar nonterminals and asks that the editor consider some other formatting convention.

Proposed resolution (June, 2010):

1. Change 8.6 [stmt.iter] paragraph 1 as follows:

Iteration statements specify looping.

iteration-statement:
while ( condition ) statement
do statement while ( expression ) ;
for ( for-init-statement condition_opt ; expression_opt ) statement
for ( for-range-declaration : expression for-range-initializer ) statement




for-init-statement:
expression-statement
simple-declaration




for-range-declaration:
type-specifier-seq attribute-specifier_opt declarator




for-range-initializer:
expression
braced-init-list

[Note: a for-init-statement ends with a semicolon. —end note]

2. Change 8.6.5 [stmt.ranged] paragraph 1 as follows:

The For a range-based for statement of the form

for ( for-range-declaration : expression ) statement

let range-init be equivalent to the expression surrounded by parentheses:

( expression )

[Footnote: this ensures that a top-level comma operator cannot be reinterpreted as a delimiter between init-declarators in the declaration of __range. —end footnote] and for a range-based for statement of the form

for ( for-range-declaration : braced-init-list ) statement

let range-init be equivalent to the braced-init-list. In each case, a range-based for statement is equivalent to

  {
    auto && __range = ( expression ) range-init;
    for ( auto __begin = begin-expr,
    ...

The intent is that the range-based for statement should be able to be used with a braced-init-list as the range over which to iterate. However, this does not work grammatically: a braced-init-list is not an expression, as required by the syntax in 8.6.5 [stmt.ranged] paragraph 1:

for ( for-range-declaration : expression ) statement

Even if this were resolved, the “equivalent to” code is not correct. It contains the declaration,

auto && __range = ( expression );

This has a similar problem, in that 9.2.9.7 [dcl.spec.auto] paragraph 3 requires that the initializer have one of the forms

= assignment-expression
( assignment-expression )

which does not allow for a braced-initializer-list. In addition, although not allowed by the grammar, 9.2.9.7 [dcl.spec.auto] paragraph 6 treats the braced-init-list specially, in order for the type deduction to work correctly:

Obtain P from T by replacing the occurrences of auto with either a new invented type template parameter U or, if the initializer is a braced-init-list (9.4.5 [dcl.init.list]), with std::initializer_list<U>.

The problem here is that a parenthesized initializer, as in the code expansion of the range-based for statement, is not a braced-init-list.