[Bug c++/109012] New: Repeated error messages when using -std=c++23

["stevenxia990430 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109012

            Bug ID: 109012
           Summary: Repeated error messages when using -std=c++23
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stevenxia990430 at gmail dot com
  Target Milestone: ---

The following incomplete program repeatedly reports the same error message on
GCC 12 with “-std=c++23” flag.

To quickly reproduce: https://gcc.godbolt.org/z/Ehzde5o4T

```
#include <iostream>

template <class T>
myClass<myClass<myClass<myClass<myClass<T>>>>> myFunction(T item)
{
    std::cout << "Hello World!";
}
```

So the C++23 parser seems to have a bug. If we remove “-std=c++23” it only
reports the concise error message once: https://gcc.godbolt.org/z/38Gxne859

Additionally, this also occurs for even syntactically invalid code snippets.
For example this code

```
myClass<myClass<myClass<myClass<myClass<int>>>>>((( myFunction(int item)
```

Also observe the same behavior, although clearly it is syntactically invalid.

This correct program compiles in less than 1 second with “g++ -c -std=c++23
correct.cpp”

```
#include <iostream>

template<typename T>
class myClass {
public:
    void myFunction(const T item);
};

template <class T>
myClass<myClass<myClass<myClass<myClass<T>>>>> myFunction(T item)
{
    std::cout << "Hello World!";
}
```
correct.cpp