[Bug c++/109887] New: Different mangled name for template specialization for clang and gcc

["yedeng.yd at linux dot alibaba.com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109887

            Bug ID: 109887
           Summary: Different mangled name for template specialization for
                    clang and gcc
           Product: gcc
           Version: 12.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: yedeng.yd at linux dot alibaba.com
  Target Milestone: ---

(This is a duplication of https://github.com/llvm/llvm-project/issues/62765
since I don't know which one is worse)

Reproducer:

```
#include <type_traits>

namespace llvm {
    class StringRef {
        public:
        StringRef(const char*);

    };
    template <typename T> class Optional {};
}

namespace n {
struct S {
    template <class T>
    std::enable_if_t<std::is_integral<T>::value, llvm::Optional<T>>
    get(llvm::StringRef) const {
        return {};
    }
};

template <>
llvm::Optional<bool>
S::get<bool>(llvm::StringRef) const;
}

void use() {
    n::S().get<bool>("hello");
}
```

For the specialization `S::get<bool>(llvm::StringRef)`, gcc will mangle it as:

```
_ZNK1n1S3getIbEENSt9enable_ifIXsrSt11is_integralIT_E5valueEN4llvm8OptionalIS4_EEE4typeENS6_9StringRefE
```

and clang will mangle it as:

```
_ZNK1n1S3getIbEENSt9enable_ifIXsr3std11is_integralIT_EE5valueEN4llvm8OptionalIS3_EEE4typeENS4_9StringRefE
```

Also the c++filt can only recognize the name mangled by gcc. And the
llvm-cxxfilt can only recognize the name mangled by clang.

So I am not sure if this is bug really or this is by design. But I think clang
and gcc are trying to make ABI compatible.