[Bug c++/101704] New: "Already defined" error in case of many concept conversion operators

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101704

            Bug ID: 101704
           Summary: "Already defined" error in case of many concept
                    conversion operators
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

Example code:
```
template <auto =[]{}> class C;
template <typename, typename = C<>> concept x = true;

#include <iostream>

struct S {
    operator x<C<>> auto() { return 42; }
    operator x<C<>> auto() { return '+'; }
    operator x<C<>> auto() { return 44LL; }
    operator x<C<>> auto() { return 45.5; }
    operator x<C<>> auto() { return 46.5f; }
    operator x<C<>> auto() { return 47L; }    
    operator x<C<>> auto() { return 48ULL; }
    operator x<C<>> auto() { return 49U; }
};

int main() {
    S s;
    std::cout << (int)s << "\n";
    std::cout << (char)s << "\n";
    std::cout << (long long)s << "\n";

    std::cout << (double)s << "\n";
    std::cout << (float)s << "\n";
    std::cout << (long)s << "\n";

    std::cout << (unsigned long long)s << "\n";
    std::cout << (unsigned)s << "\n";
}
```

GCC 11 prints:
```
/tmp/ccfcrhcg.s: Assembler messages:
/tmp/ccfcrhcg.s:33: Error: symbol `_ZN1ScvDaEv' is already defined
/tmp/ccfcrhcg.s:55: Error: symbol `_ZN1ScvDaEv' is already defined
/tmp/ccfcrhcg.s:77: Error: symbol `_ZN1ScvDaEv' is already defined
/tmp/ccfcrhcg.s:101: Error: symbol `_ZN1ScvDaEv' is already defined
/tmp/ccfcrhcg.s:123: Error: symbol `_ZN1ScvDaEv' is already defined
/tmp/ccfcrhcg.s:145: Error: symbol `_ZN1ScvDaEv' is already defined
/tmp/ccfcrhcg.s:167: Error: symbol `_ZN1ScvDaEv' is already defined
Execution build compiler returned: 1
```

GCC (truck) crashes on this example:
```
<source>:29:1: internal compiler error: symtab_node::verify failed
0x1dabd49 internal_error(char const*, ...)
        ???:0
0xb68322 symtab_node::verify_symtab_nodes()
        ???:0
0xb844cf symbol_table::finalize_compilation_unit()
        ???:0
```

https://gcc.godbolt.org/z/KP8Wqhz96