std::basic_string<_Tp> constructor point of instantiation woes?

[Stephan Bergmann <sbergman@redhat.com>]

When using recent libstc++ trunk with Clang in C++20 mode, 
std::u16string literals as in

> #include <string>
> int main() {
>   using namespace std::literals;
>   u""s;
> }

started to cause linker failures due to undefined

> _ZNSt7__cxx1112basic_stringIDsSt11char_traitsIDsESaIDsEE12_M_constructIPKDsEEvT_S8_St20forward_iterator_tag

After some head scratching, I found the more insightful

> $ cat test.cc
> #include <string>
> constexpr std::string s("", 0);

> $ clang++ -std=c++20 -fsyntax-only test.cc
> test.cc:2:23: error: constexpr variable 's' must be initialized by a constant expression
> constexpr std::string s("", 0);
>                       ^~~~~~~~
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/12.0.0/../../../../include/c++/12.0.0/bits/basic_string.h:620:2: note: undefined function '_M_construct<const char *>' cannot be used in a constant expression
>         _M_construct(__s, __s + __n, std::forward_iterator_tag());
>         ^
> test.cc:2:23: note: in call to 'basic_string(&""[0], 0, std::allocator<char>())'
> constexpr std::string s("", 0);
>                       ^
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/12.0.0/../../../../include/c++/12.0.0/bits/basic_string.h:331:9: note: declared here
>         _M_construct(_FwdIterator __beg, _FwdIterator __end,
>         ^
> 1 error generated.

and after some more head scratching found Clang to complain about the 
reduced

> template<typename> struct S {
>     constexpr void f();
>     constexpr S() { f(); };
> };
> S<void> s1;
> template<typename T> constexpr void S<T>::f() {}
> constexpr S<void> s2;

(about which GCC does not complain).  Not entirely sure who is right, 
but what would help Clang is to move the definitions of the literal 
operators in basic_string.h (which implicitly instantiate the 
corresponding std::basic_string<_Tp> constructor) past the definition of 
_M_construct (which is called from the constructor) in basic_string.tcc; 
something like

> diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> index 6e7de738308..871ca89e16e 100644
> --- a/libstdc++-v3/include/bits/basic_string.h
> +++ b/libstdc++-v3/include/bits/basic_string.h
> @@ -4304,55 +4304,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      struct __is_fast_hash<hash<u32string>> : std::false_type
>      { };
>  
> -#if __cplusplus >= 201402L
> -
> -#define __cpp_lib_string_udls 201304
> -
> -  inline namespace literals
> -  {
> -  inline namespace string_literals
> -  {
> -#pragma GCC diagnostic push
> -#pragma GCC diagnostic ignored "-Wliteral-suffix"
> -
> -#if __cpp_lib_constexpr_string >= 201907L
> -# define _GLIBCXX_STRING_CONSTEXPR constexpr
> -#else
> -# define _GLIBCXX_STRING_CONSTEXPR
> -#endif
> -
> -    _GLIBCXX_DEFAULT_ABI_TAG _GLIBCXX_STRING_CONSTEXPR
> -    inline basic_string<char>
> -    operator""s(const char* __str, size_t __len)
> -    { return basic_string<char>{__str, __len}; }
> -
> -    _GLIBCXX_DEFAULT_ABI_TAG _GLIBCXX_STRING_CONSTEXPR
> -    inline basic_string<wchar_t>
> -    operator""s(const wchar_t* __str, size_t __len)
> -    { return basic_string<wchar_t>{__str, __len}; }
> -
> -#ifdef _GLIBCXX_USE_CHAR8_T
> -    _GLIBCXX_DEFAULT_ABI_TAG _GLIBCXX_STRING_CONSTEXPR
> -    inline basic_string<char8_t>
> -    operator""s(const char8_t* __str, size_t __len)
> -    { return basic_string<char8_t>{__str, __len}; }
> -#endif
> -
> -    _GLIBCXX_DEFAULT_ABI_TAG _GLIBCXX_STRING_CONSTEXPR
> -    inline basic_string<char16_t>
> -    operator""s(const char16_t* __str, size_t __len)
> -    { return basic_string<char16_t>{__str, __len}; }
> -
> -    _GLIBCXX_DEFAULT_ABI_TAG _GLIBCXX_STRING_CONSTEXPR
> -    inline basic_string<char32_t>
> -    operator""s(const char32_t* __str, size_t __len)
> -    { return basic_string<char32_t>{__str, __len}; }
> -
> -#undef _GLIBCXX_STRING_CONSTEXPR
> -#pragma GCC diagnostic pop
> -  } // inline namespace string_literals
> -  } // inline namespace literals
> -
>  #if __cplusplus >= 201703L
>    namespace __detail::__variant
>    {
> @@ -4369,7 +4320,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        { };
>    }  // namespace __detail::__variant
>  #endif // C++17
> -#endif // C++14
>  
>  _GLIBCXX_END_NAMESPACE_VERSION
>  } // namespace std
> diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
> index 6f619a08f70..2fd607ef50a 100644
> --- a/libstdc++-v3/include/bits/basic_string.tcc
> +++ b/libstdc++-v3/include/bits/basic_string.tcc
> @@ -1123,6 +1123,57 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  #endif // _GLIBCXX_USE_WCHAR_T
>  #endif // _GLIBCXX_EXTERN_TEMPLATE
>  
> +#if __cplusplus >= 201402L
> +
> +#define __cpp_lib_string_udls 201304
> +
> +  inline namespace literals
> +  {
> +  inline namespace string_literals
> +  {
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wliteral-suffix"
> +
> +#if __cpp_lib_constexpr_string >= 201907L
> +# define _GLIBCXX_STRING_CONSTEXPR constexpr
> +#else
> +# define _GLIBCXX_STRING_CONSTEXPR
> +#endif
> +
> +    _GLIBCXX_DEFAULT_ABI_TAG _GLIBCXX_STRING_CONSTEXPR
> +    inline basic_string<char>
> +    operator""s(const char* __str, size_t __len)
> +    { return basic_string<char>{__str, __len}; }
> +
> +    _GLIBCXX_DEFAULT_ABI_TAG _GLIBCXX_STRING_CONSTEXPR
> +    inline basic_string<wchar_t>
> +    operator""s(const wchar_t* __str, size_t __len)
> +    { return basic_string<wchar_t>{__str, __len}; }
> +
> +#ifdef _GLIBCXX_USE_CHAR8_T
> +    _GLIBCXX_DEFAULT_ABI_TAG _GLIBCXX_STRING_CONSTEXPR
> +    inline basic_string<char8_t>
> +    operator""s(const char8_t* __str, size_t __len)
> +    { return basic_string<char8_t>{__str, __len}; }
> +#endif
> +
> +    _GLIBCXX_DEFAULT_ABI_TAG _GLIBCXX_STRING_CONSTEXPR
> +    inline basic_string<char16_t>
> +    operator""s(const char16_t* __str, size_t __len)
> +    { return basic_string<char16_t>{__str, __len}; }
> +
> +    _GLIBCXX_DEFAULT_ABI_TAG _GLIBCXX_STRING_CONSTEXPR
> +    inline basic_string<char32_t>
> +    operator""s(const char32_t* __str, size_t __len)
> +    { return basic_string<char32_t>{__str, __len}; }
> +
> +#undef _GLIBCXX_STRING_CONSTEXPR
> +#pragma GCC diagnostic pop
> +  } // inline namespace string_literals
> +  } // inline namespace literals
> +
> +#endif // C++14
> +
>  _GLIBCXX_END_NAMESPACE_VERSION
>  } // namespace std
>