libstdc++: Enforce value_type consistency in strings and streams

P1463R1 made it ill-formed for allocator-aware containers (including
std::basic_string) to use an allocator that has a different value_type
from the container itself. We already enforce that for other containers
(since r8-4828-g866e4d3853ccc0), but not for std::basic_string. We
traditionally accepted it as an extension and rebound the allocator, so
this change only adds the enforcement for C++20 and later.

Similarly, P1148R0 made it ill-formed for strings and streams to use a
traits type that has an incorrect char_type. We already enforce that for
std::basic_string_view, so we just need to add it to std::basic_ios and
std::basic_string.

The assertion for the allocator's value_type caused some testsuite
regressions:
FAIL: 21_strings/basic_string/cons/char/deduction.cc (test for excess errors)
FAIL: 21_strings/basic_string/cons/wchar_t/deduction.cc (test for excess errors)
FAIL: 21_strings/basic_string/requirements/explicit_instantiation/debug.cc (test for excess errors)
FAIL: 21_strings/basic_string/requirements/explicit_instantiation/int.cc (test for excess errors)

The last two are testing the traditional extension that rebinds the
allocator, so need to be disabled for C++20.

The first two are similar to LWG 3076 where an incorrect constructor is
considered for CTAD. In this case, determining that it's not viable
requires instantiating std::basic_string<Iter, char_traits<Iter>, Alloc>
which then fails the new assertion, because Alloc::value_type is not the
same as Iter. This is only a problem because the size_type parameter of
the non-viable constructor is an alias for
_Alloc_traits_impl<A>::size_type which is a nested type, and so the
enclosing basic_string specialization needs to be instantiated. If we
remove the _Alloc_traits_impl wrapper that was added in
r12-5413-g2d76292bd6719d, then the definition of size_type no longer
depends on basic_string, and we don't instantiate an invalid
specialization and don't fail the assertion. The work done by
_Alloc_traits_impl::allocate can be done in a _S_allocate function
instead, which is probably more efficient to compile anyway.

libstdc++-v3/ChangeLog:

	* config/abi/pre/gnu.ver: Export basic_string::_S_allocate.
	* include/bits/basic_ios.h: Add static assertion checking
	traits_type::value_type.
	* include/bits/basic_string.h: Likewise. Do not rebind
	allocator, and add static assertion checking its value_type.
	(basic_string::_Alloc_traits_impl): Remove class template.
	(basic_string::_S_allocate): New static member function.
	(basic_string::assign): Use _S_allocate.
	* include/bits/basic_string.tcc (basic_string::_M_create)
	(basic_string::reserve, basic_string::_M_replace): Likewise.
	* testsuite/21_strings/basic_string/requirements/explicit_instantiation/debug.cc:
	Disable for C++20 and later.
	* testsuite/21_strings/basic_string/requirements/explicit_instantiation/int.cc:
	Likweise.

diff --git a/libstdc++-v3/config/abi/pre/gnu.ver b/libstdc++-v3/config/abi/pre/gnu.ver
index 36bb87880d7f117f4e28ea8c6276e5c6799d6280..768cd4a4a6cdd7c4c439a0c39e2e8630ab540d14 100644 (file)
--- a/libstdc++-v3/config/abi/pre/gnu.ver
+++ b/libstdc++-v3/config/abi/pre/gnu.ver
@@ -1759,7 +1759,9 @@ GLIBCXX_3.4.21 {
 #endif
 
     # ABI-tagged std::basic_string
-    _ZNSt7__cxx1112basic_stringI[cw]St11char_traitsI[cw]ESaI[cw]EE1[01]**;
+    _ZNSt7__cxx1112basic_stringI[cw]St11char_traitsI[cw]ESaI[cw]EE10_M_[dr]*;
+    _ZNSt7__cxx1112basic_stringI[cw]St11char_traitsI[cw]ESaI[cw]EE10_S_compareE[jmy][jmy];
+    _ZNSt7__cxx1112basic_stringI[cw]St11char_traitsI[cw]ESaI[cw]EE11_M_capacityE[jmy];
     _ZNSt7__cxx1112basic_stringI[cw]St11char_traitsI[cw]ESaI[cw]EE12_Alloc_hiderC[12]EP[cw]RKS3_;
     _ZNSt7__cxx1112basic_stringI[cw]St11char_traitsI[cw]ESaI[cw]EE12_M*;
     _ZNSt7__cxx1112basic_stringI[cw]St11char_traitsI[cw]ESaI[cw]EE13*;
@@ -2516,6 +2518,7 @@ GLIBCXX_3.4.31 {
 
 GLIBCXX_3.4.32 {
     _ZSt21ios_base_library_initv;
+    _ZNSt7__cxx1112basic_stringI[cw]St11char_traitsI[cw]ESaI[cw]EE11_S_allocateERS3_[jmy];
 } GLIBCXX_3.4.31;
 
 # Symbols in the support library (libsupc++) have their own tag.

diff --git a/libstdc++-v3/include/bits/basic_ios.h b/libstdc++-v3/include/bits/basic_ios.h
index de5719c1d68bc0e3fcc67d5dfa2f1c1df95d0159..c7c391c0f49cf1534f79d4170d13a892ef809cbc 100644 (file)
--- a/libstdc++-v3/include/bits/basic_ios.h
+++ b/libstdc++-v3/include/bits/basic_ios.h
@@ -66,6 +66,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _CharT, typename _Traits>
     class basic_ios : public ios_base
     {
+#if __cplusplus >= 202002L
+      static_assert(is_same_v<_CharT, typename _Traits::char_type>);
+#endif
+
     public:
       ///@{
       /**

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index b16b2898b62a05aa58151df350ed43b830b9accc..d15f0f0589f0a3edbfd122f85c4bc731884a6ed3 100644 (file)
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -86,39 +86,16 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
   template<typename _CharT, typename _Traits, typename _Alloc>
     class basic_string
     {
+#if __cplusplus >= 202002L
+      static_assert(is_same_v<_CharT, typename _Traits::char_type>);
+      static_assert(is_same_v<_CharT, typename _Alloc::value_type>);
+      using _Char_alloc_type = _Alloc;
+#else
       typedef typename __gnu_cxx::__alloc_traits<_Alloc>::template
        rebind<_CharT>::other _Char_alloc_type;
+#endif
 
-#if __cpp_lib_constexpr_string < 201907L
       typedef __gnu_cxx::__alloc_traits<_Char_alloc_type> _Alloc_traits;
-#else
-      template<typename _Traits2, typename _Dummy_for_PR85282>
-       struct _Alloc_traits_impl : __gnu_cxx::__alloc_traits<_Char_alloc_type>
-       {
-         typedef __gnu_cxx::__alloc_traits<_Char_alloc_type> _Base;
-
-         [[__gnu__::__always_inline__]]
-         static constexpr typename _Base::pointer
-         allocate(_Char_alloc_type& __a, typename _Base::size_type __n)
-         {
-           pointer __p = _Base::allocate(__a, __n);
-           if (std::is_constant_evaluated())
-             // Begin the lifetime of characters in allocated storage.
-             for (size_type __i = 0; __i < __n; ++__i)
-               std::construct_at(__builtin_addressof(__p[__i]));
-           return __p;
-         }
-       };
-
-      template<typename _Dummy_for_PR85282>
-       struct _Alloc_traits_impl<char_traits<_CharT>, _Dummy_for_PR85282>
-       : __gnu_cxx::__alloc_traits<_Char_alloc_type>
-       {
-         // std::char_traits begins the lifetime of characters.
-       };
-
-      using _Alloc_traits = _Alloc_traits_impl<_Traits, void>;
-#endif
 
       // Types:
     public:
@@ -149,6 +126,22 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 #endif
 
     private:
+      static _GLIBCXX20_CONSTEXPR pointer
+      _S_allocate(_Char_alloc_type& __a, size_type __n)
+      {
+       pointer __p = _Alloc_traits::allocate(__a, __n);
+#if __cpp_lib_constexpr_string >= 201907L
+       // std::char_traits begins the lifetime of characters,
+       // but custom traits might not, so do it here.
+       if constexpr (!is_same_v<_Traits, char_traits<_CharT>>)
+         if (std::__is_constant_evaluated())
+           // Begin the lifetime of characters in allocated storage.
+           for (size_type __i = 0; __i < __n; ++__i)
+             std::construct_at(__builtin_addressof(__p[__i]));
+#endif
+       return __p;
+      }
+
 #if __cplusplus >= 201703L
       // A helper type for avoiding boiler-plate.
       typedef basic_string_view<_CharT, _Traits> __sv_type;
@@ -1596,7 +1589,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
                    const auto __len = __str.size();
                    auto __alloc = __str._M_get_allocator();
                    // If this allocation throws there are no effects:
-                   auto __ptr = _Alloc_traits::allocate(__alloc, __len + 1);
+                   auto __ptr = _S_allocate(__alloc, __len + 1);
                    _M_destroy(_M_allocated_capacity);
                    _M_data(__ptr);
                    _M_capacity(__len);

diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 99fdbeee5ad95092d170af27c4bc01f091cde6dc..d8a279fc9edbe0a7c2239c75637717f9fc0cb451 100644 (file)
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -152,7 +152,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       // NB: Need an array of char_type[__capacity], plus a terminating
       // null char_type() element.
-      return _Alloc_traits::allocate(_M_get_allocator(), __capacity + 1);
+      return _S_allocate(_M_get_allocator(), __capacity + 1);
     }
 
   // NB: This is the special case for Input Iterators, used in
@@ -376,8 +376,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       else if (__length < __capacity)
        try
          {
-           pointer __tmp
-             = _Alloc_traits::allocate(_M_get_allocator(), __length + 1);
+           pointer __tmp = _S_allocate(_M_get_allocator(), __length + 1);
            this->_S_copy(__tmp, _M_data(), __length + 1);
            _M_dispose();
            _M_data(__tmp);
@@ -521,8 +520,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #if __cpp_lib_is_constant_evaluated
          if (std::is_constant_evaluated())
            {
-             auto __newp = _Alloc_traits::allocate(_M_get_allocator(),
-                                                   __new_size);
+             auto __newp = _S_allocate(_M_get_allocator(), __new_size);
              _S_copy(__newp, this->_M_data(), __pos);
              _S_copy(__newp + __pos, __s, __len2);
              _S_copy(__newp + __pos + __len2, __p + __len1, __how_much);

diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/requirements/explicit_instantiation/debug.cc b/libstdc++-v3/testsuite/21_strings/basic_string/requirements/explicit_instantiation/debug.cc
index b70fbd504604a70692f1717d993dfbbc394200d0..6bb46c5356ce9267925e57df4b0657c85d082cdf 100644 (file)
--- a/libstdc++-v3/testsuite/21_strings/basic_string/requirements/explicit_instantiation/debug.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/requirements/explicit_instantiation/debug.cc
@@ -19,7 +19,7 @@
 
 #include <debug/string>
 
-// { dg-do compile }
+// { dg-do compile { target c++17_down } }
 
 // libstdc++/21770
 namespace debug = __gnu_debug;

diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/requirements/explicit_instantiation/int.cc b/libstdc++-v3/testsuite/21_strings/basic_string/requirements/explicit_instantiation/int.cc
index 8326278b36c4a910ef43264fbfebc46863abda7b..4f23cc4dcefef546b68e8feebada29f8a9ae0bb6 100644 (file)
--- a/libstdc++-v3/testsuite/21_strings/basic_string/requirements/explicit_instantiation/int.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/requirements/explicit_instantiation/int.cc
@@ -20,7 +20,7 @@
 
 #include <string>
 
-// { dg-do compile }
+// { dg-do compile { target c++17_down } }
 
 // libstdc++/21770
 template class std::basic_string<int, std::char_traits<int>,