CWG2571 [expr.sub] CWG2507 doesn't specify the evaluation order for default arguments

[cor3ntin]

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

Corentin Jabot

Reference (section label):

[expr.sub]

Link to reflector thread (if any):

https://lists.isocpp.org/core/2022/04/12363.php

Issue description:

While CWG2507 add the missing wording for defaults arguments in [over.sub], it doesn't specify their evaluation order
relative to the postfix-expression.
For consistency with call expressions - and because it was not suggested it should be changed - the expressions (in the expression lists) evaluation order is left indeterminate.

It is unclear to me whether [over.match.oper] needs further massaging to cover the evaluation order for a subscript expression with not exactly one expression in its expression-list.

Suggested resolution:

Modify 7.6.1.2 [expr.sub]/p1

A [subscript expression is a postfix expression followed by square brackets containing a possibly empty, 
comma-separated list of [initializer-clause]s which constitute the arguments to the subscript operator.
The [postfix-expression] is sequenced before each expression in the [expression-list]
+ and any default argument
. 

[jensmaurer]

CWG2571

The suggested resolution is a bit different.