[Bug c++/125537] [contracts] g++-16 crashes on contracts pre and post with placeholder return type

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125537

--- Comment #6 from Wang Jinghao <zheng.xianyuwang at gmail dot com> ---
(In reply to Waffl3x from comment #5)
> ```
> bool g(int) { return true; }
> 
> template<typename T>
> auto f(T a)
>   pre(g(a)) {}
> 
> template auto f<int>(int);
> ```
> Reproducer without member functions.

The trigger is a type conversion in a pre/post condition within a function
template whose return type is auto.