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);
```
