[libc++] Remove the std::string base class

Removing the base class of std::basic_string is not an ABI break, so we can remove any references to it from the header.

Reviewed By: ldionne, Mordante, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D118733

libcxx/include/__config

@@ -115,7 +115,7 @@
 // about ABI compatibility.
 #  define _LIBCPP_ABI_NO_RANDOM_DEVICE_COMPATIBILITY_LAYOUT
 // Remove basic_string common base
-#  define _LIBCPP_ABI_NO_BASIC_STRING_BASE_CLASS
+#  define _LIBCPP_ABI_DO_NOT_EXPORT_BASIC_STRING_COMMON
 #elif _LIBCPP_ABI_VERSION == 1
 #  if !defined(_LIBCPP_OBJECT_FORMAT_COFF)
 // Enable compiling copies of now inline methods into the dylib to support

libcxx/include/string

@@ -618,18 +618,6 @@ operator+(const basic_string<_CharT, _Traits, _Allocator>& __x, _CharT __y);
 
 _LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS string operator+<char, char_traits<char>, allocator<char> >(char const*, string const&))
 
-#ifndef _LIBCPP_ABI_NO_BASIC_STRING_BASE_CLASS
-template <bool>
-struct __basic_string_common;
-
-template <>
-struct __basic_string_common<true> {
-    // Both are defined in string.cpp
-    _LIBCPP_NORETURN _LIBCPP_EXPORTED_FROM_ABI void __throw_length_error() const;
-    _LIBCPP_NORETURN _LIBCPP_EXPORTED_FROM_ABI void __throw_out_of_range() const;
-};
-#endif
-
 template <class _Iter>
 struct __string_is_trivial_iterator : public false_type {};
 
@@ -682,9 +670,6 @@ class
     _LIBCPP_PREFERRED_NAME(u32string)
 #endif
     basic_string
-#ifndef _LIBCPP_ABI_NO_BASIC_STRING_BASE_CLASS
-    : private __basic_string_common<true> // This base class is historical, but it needs to remain for ABI compatibility
-#endif
 {
 public:
     typedef basic_string                                 __self;

libcxx/src/string.cpp

@@ -21,15 +21,26 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#ifndef _LIBCPP_ABI_NO_BASIC_STRING_BASE_CLASS
+#ifndef _LIBCPP_ABI_DO_NOT_EXPORT_BASIC_STRING_COMMON
+
+template <bool>
+struct __basic_string_common;
+
+// The struct isn't declared anymore in the headers. It's only here for ABI compatibility.
+template <>
+struct __basic_string_common<true> {
+    _LIBCPP_NORETURN _LIBCPP_EXPORTED_FROM_ABI void __throw_length_error() const;
+    _LIBCPP_NORETURN _LIBCPP_EXPORTED_FROM_ABI void __throw_out_of_range() const;
+};
+
 void __basic_string_common<true>::__throw_length_error() const {
-    _VSTD::__throw_length_error("basic_string");
+    std::__throw_length_error("basic_string");
 }
-
 void __basic_string_common<true>::__throw_out_of_range() const {
-    _VSTD::__throw_out_of_range("basic_string");
+    std::__throw_out_of_range("basic_string");
 }
-#endif
+
+#endif // _LIBCPP_ABI_DO_NOT_EXPORT_BASIC_STRING_COMMON
 
 #define _LIBCPP_EXTERN_TEMPLATE_DEFINE(...) template __VA_ARGS__;
 #ifdef _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATION