[Bug c++/127196] [contracts] redeclaration of function with type dependent param used in postcondition accepts non-const param

[waffl3x at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127196

--- Comment #1 from Waffl3x <waffl3x at gcc dot gnu.org> ---
https://godbolt.org/z/T76asK9nY
```
template<typename T>
bool g();

template<>
bool g<int>() { __builtin_printf("Shouldn't happen"); return true; }
template<>
bool g<const int>() { return true; }

template<typename T>
void f0(T a)
  post(g<decltype(a)>());

template<typename T>
void f0(T const a) {}

template void f0<int>(int);


template<typename T>
void f1(T const a)
  post(g<decltype(a)>()) {}

template void f1<int>(int);

int main()
{
  f1(42);
  f0(42);
}
```
Unfortunately this is not merely a diagnostic issue.
I'm working on this.