[libc++] Use always_inline for std::stringbuf::str() on Windows to work around #40363

[hans]

Summary

As reported in https://github.com/llvm/llvm-project/issues/40363, the exclude_from_explicit_instantiation attribute doesn't currently really work with dllimport.

Because of that, the new stringbuf::str() overload added in D153709 is dllimport on Windows but not actually defined in the cpp file, causing link errors.

As discussed on that code review, this patch works around the problem with the always_inline attribute until Clang can be fixed.

[pfusik]

I think https://reviews.llvm.org/D155185 addresses it better: there are two overloads to handle for now, there will be eight.

[hans]

In D155180#4497048, @pfusik wrote:

I think https://reviews.llvm.org/D155185 addresses it better: there are two overloads to handle for now, there will be eight.

That does look better :) Thanks!

libcxx/include/sstream

 
 #if _LIBCPP_STD_VER >= 20
     _LIBCPP_HIDE_FROM_ABI allocator_type get_allocator() const noexcept { return __str_.get_allocator(); }
 #endif
 
 #if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_BUILDING_LIBRARY)
     string_type str() const;
 #else
-    _LIBCPP_HIDE_FROM_ABI string_type str() const & { return str(__str_.get_allocator()); }
+
+#ifdef _WIN32
+    // TODO(LLVM 19): Remove this once we drop support for Clang versions which
+    // have this bug: https://github.com/llvm/llvm-project/issues/40363
+    _LIBCPP_ALWAYS_INLINE
+#else
+    _LIBCPP_HIDE_FROM_ABI
+#endif
+    string_type str() const & { return str(__str_.get_allocator()); }
 
     template <class _SAlloc>
       requires __is_allocator<_SAlloc>::value
     _LIBCPP_HIDE_FROM_ABI basic_string<char_type, traits_type, _SAlloc> str(const _SAlloc& __sa) const {
         return basic_string<_CharT, _Traits, _SAlloc>(view(), __sa);
     }
 
     _LIBCPP_HIDE_FROM_ABI string_type str() && {