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libstdc++: Limit new basic_string(nullptr_t) constructor to C++23 [PR104099]

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The new deleted constructors added by P2166R1 are a breaking change,
making previously valid code ill-formed in C++23. As a result, they
should only be defined for C++23 and not for C++11 and up.

libstdc++-v3/ChangeLog:

	PR libstdc++/104099
	* include/bits/basic_string.h (basic_string(nullptr_t)): Only
	define for C++23.
	(operator=(nullptr_t)): Likewise.
	* include/bits/cow_string.h: Likewise.
	* include/std/string_view (basic_string_view(nullptr_t)):
	Likewise.
	* testsuite/21_strings/basic_string/cons/char/nullptr.cc: Adjust
	expected error. Add examples that become ill-formed in C++23.
	* testsuite/21_strings/basic_string_view/cons/char/nonnull.cc:
	Adjust expected errors.
	* testsuite/21_strings/basic_string_view/cons/wchar_t/nonnull.cc:
	Likewise.
This commit is contained in:
Jonathan Wakely 2022-01-18 16:26:45 +00:00
parent 3c4a54adb2
commit fe3ed885cd
6 changed files with 37 additions and 6 deletions
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4
libstdc++-v3/include/bits/basic_string.h
View File

@ -728,10 +728,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
else
_M_construct(__str.begin(), __str.end(), std::forward_iterator_tag());
}
#endif // C++11
#if __cplusplus >= 202100L
basic_string(nullptr_t) = delete;
basic_string& operator=(nullptr_t) = delete;
#endif // C++11
#endif // C++23
/**
* @brief Construct string as copy of a range.

4
libstdc++-v3/include/bits/cow_string.h
View File

@ -665,10 +665,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
else
_M_dataplus._M_p = _S_construct(__str.begin(), __str.end(), __a);
}
#endif // C++11
#if __cplusplus >= 202100L
basic_string(nullptr_t) = delete;
basic_string& operator=(nullptr_t) = delete;
#endif // C++11
#endif // C++23
/**
* @brief Construct string as copy of a range.

3
libstdc++-v3/include/std/string_view
View File

@ -167,10 +167,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
noexcept(noexcept(ranges::size(__r)) && noexcept(ranges::data(__r)))
: _M_len(ranges::size(__r)), _M_str(ranges::data(__r))
{ }
basic_string_view(nullptr_t) = delete;
#endif // C++23
#endif // C++20
basic_string_view(nullptr_t) = delete;
constexpr basic_string_view&
operator=(const basic_string_view&) noexcept = default;

26
libstdc++-v3/testsuite/21_strings/basic_string/cons/char/nullptr.cc
View File

@ -1,4 +1,28 @@
// { dg-do compile { target c++11 } }
#include <string>
std::string s = nullptr; // { dg-error "deleted" "P2166R1" }
std::string s = nullptr; // { dg-error "deleted" "P2166R1" { target c++23 } }
struct S
{
operator const char*() const { return ""; }
operator std::nullptr_t() const { return {}; }
};
std::string s2{ S{} }; // { dg-error "deleted" "PR 104099" { target c++23 } }
#if __cpp_concepts
struct J
{
// In C++20 this selects basic_string(const char*),
// in C++23 it's ambiguous due to basic_string(nullptr_t).
template<typename T>
requires (!std::is_same_v<std::allocator<char>, T>)
&& (!std::is_same_v<std::string, T>)
&& (!std::is_same_v<char, T>)
&& (!std::is_same_v<std::string_view, T>)
operator T() const { return {}; }
};
std::string s3{ J{} }; // { dg-error "ambiguous" "PR 104099" { target c++23 } }
#endif

3
libstdc++-v3/testsuite/21_strings/basic_string_view/cons/char/nonnull.cc
View File

@ -25,5 +25,6 @@ test01()
{
std::string_view s((const char*)nullptr); // { dg-warning "\\\[-Wnonnull" }
std::string_view t((char*)nullptr); // { dg-warning "\\\[-Wnonnull" }
std::string_view u(nullptr); // { dg-error "deleted" }
std::string_view u(nullptr); // { dg-warning "\\\[-Wnonnull" "" { target c++20_down } }
// { dg-error "deleted" "P2166R1" { target c++23 } 0 }
}

3
libstdc++-v3/testsuite/21_strings/basic_string_view/cons/wchar_t/nonnull.cc
View File

@ -25,5 +25,6 @@ test01()
{
std::wstring_view s((const wchar_t*)nullptr); // { dg-warning "\\\[-Wnonnull" }
std::wstring_view t((wchar_t*)nullptr); // { dg-warning "\\\[-Wnonnull" }
std::wstring_view u(nullptr); // { dg-error "deleted" }
std::wstring_view u(nullptr); // { dg-warning "\\\[-Wnonnull" "" { target c++20_down } }
// { dg-error "deleted" "P2166R1" { target c++23 } 0 }
}