[Bug c++/126894] New: [contracts] Incorrectly mark a parameter as ODR-used when a pack indexing expression is used in a postcondition.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126894

            Bug ID: 126894
           Summary: [contracts] Incorrectly mark a parameter as ODR-used
                    when a pack indexing expression is used in a
                    postcondition.
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: c++-contracts
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zheng.xianyuwang at gmail dot com
  Target Milestone: ---

Consider the following example:

```
template <typename... Args>
  void f (Args... args)
    post (args...[1])
  {}


template void f<int, int&> (int, int&);
```

In the postcondition, only `args...[1]' is ODR-used, so when checking the rule
that “a non-reference parameter used in a postcondition must have const type”,
we should likewise only check `args...[1]'.

The current trunk build incorrectly diagnoses:

`error: value parameter 'args#0' used in a postcondition must be const'

This is probably because, in cp_parser_primary_expression, finish_id_expression
-> check_param_in_postcondition marks `args' as used-in-post before
cp_parser_pack_index processes the pack indexing expression. This then causes
check_postconditions_in_redecl to issue the erroneous diagnostic.

Since check_postconditions_in_redecl on trunk also has a one-to-many mapping
problem when instantiating parameter packs (PR124395), the following case is
still missing a diagnostic:

```
extern template void f<int&, int> (int&, int);
```