[libc++] Fix link error with _LIBCPP_HIDE_FROM_ABI_PER_TU and std::string

[ldionne]

Summary

This is effectively a revert of r344616, which was a partial fix for
PR38964 (compilation of <string> with GCC in C++03 mode). However, that
configuration is explicitly not supported anymore and that partial fix
breaks compilation with Clang when per-TU insulation is provided.

PR42676
rdar://52899715

[mclow.lists]

The first three removals are fine.
I don't remember why I put the last two in, but I guess I'm OK with them.

[EricWF]

This change isn't strictly correct. A lot of these constructors were externally instantiated before we made them templates.
I suspect this will break the ABI.

We'll need to wrap these in _LIBCPP_BUILDING_LIBRARY to continue generating the old definitions.

[EricWF]

@ldionne pointed out to me that I'm all kinds of wrong.

LGTM. Sorry about the confusion.

libcxx/trunk/include/string

 #else
         _NOEXCEPT;
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
     basic_string(basic_string&& __str, const allocator_type& __a);
 #endif  // _LIBCPP_CXX03_LANG
 
-#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
     template <class = typename enable_if<__is_allocator<_Allocator>::value, nullptr_t>::type>
-#endif
     _LIBCPP_INLINE_VISIBILITY
     basic_string(const _CharT* __s) {
       _LIBCPP_ASSERT(__s != nullptr, "basic_string(const char*) detected nullptr");
       __init(__s, traits_type::length(__s));
 #   if _LIBCPP_DEBUG_LEVEL >= 2
       __get_db()->__insert_c(this);
 #   endif
     }
 
-#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
     template <class = typename enable_if<__is_allocator<_Allocator>::value, nullptr_t>::type>
-#endif
         _LIBCPP_INLINE_VISIBILITY
         basic_string(const _CharT* __s, const _Allocator& __a);
 
     _LIBCPP_INLINE_VISIBILITY
     basic_string(const _CharT* __s, size_type __n);
     _LIBCPP_INLINE_VISIBILITY
     basic_string(const _CharT* __s, size_type __n, const _Allocator& __a);
     _LIBCPP_INLINE_VISIBILITY
     basic_string(size_type __n, _CharT __c);
 
-#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
     template <class = typename enable_if<__is_allocator<_Allocator>::value, nullptr_t>::type>
-#endif
         _LIBCPP_INLINE_VISIBILITY
         basic_string(size_type __n, _CharT __c, const _Allocator& __a);
 
     basic_string(const basic_string& __str, size_type __pos, size_type __n,
                  const _Allocator& __a = _Allocator());
     _LIBCPP_INLINE_VISIBILITY
     basic_string(const basic_string& __str, size_type __pos,
                  const _Allocator& __a = _Allocator());
else
         __set_long_cap(__cap+1);
         __set_long_size(__sz);
     }
     traits_type::copy(_VSTD::__to_raw_pointer(__p), __s, __sz);
     traits_type::assign(__p[__sz], value_type());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
 template <class>
-#endif
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const _CharT* __s, const _Allocator& __a)
     : __r_(__second_tag(), __a)
 {
     _LIBCPP_ASSERT(__s != nullptr, "basic_string(const char*, allocator) detected nullptr");
     __init(__s, traits_type::length(__s));
 #if _LIBCPP_DEBUG_LEVEL >= 2
     __get_db()->__insert_c(this);
 #endif

 {
     __init(__n, __c);
 #if _LIBCPP_DEBUG_LEVEL >= 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
 template <class>
-#endif
 basic_string<_CharT, _Traits, _Allocator>::basic_string(size_type __n, _CharT __c, const _Allocator& __a)
     : __r_(__second_tag(), __a)
 {
     __init(__n, __c);
 #if _LIBCPP_DEBUG_LEVEL >= 2
     __get_db()->__insert_c(this);
 #endif
 }

libcxx/trunk/test/libcxx/strings/basic.string/PR42676.sh.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// Regression test for PR42676.
+
+// RUN: %cxx %flags %s -o %t.exe %compile_flags -D_LIBCPP_HIDE_FROM_ABI_PER_TU
+// RUN: %t.exe
+
+#include <memory>
+#include <string>
+
+int main() {
+    std::string s1(10u, '-', std::allocator<char>()); (void)s1;
+    std::string s2("hello", std::allocator<char>()); (void)s2;
+    std::string s3("hello"); (void)s3;
+}