libstdc++: Check feature test macro for std::string_view in <string>

We can use the __glibcxx_string_view macro to guard the uses of
std::string_view in <string>, instead of just checking the value of
__cplusplus. It makes no practical difference because
__glibcxx_string_view is defined for C++17 and up, but it makes it clear
to readers that the lines guarded by that macro are features that depend
on string_view.

We could be more precise and check __glibcxx_string_view >= 201606L
which is the value for the P0254R2 paper that integrated
std::string_view with std::string, but I think just checking for the
macro being defined is clear enough.

We can also check __glibcxx_variant for the _Never_valueless_alt partial
specialization.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h: Check __glibcxx_string_view and
	__glibcxx_variant instead of __cplusplus >= 2017L.
	* include/bits/cow_string.h: Likewise.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

2 files changed, 35 insertions, 34 deletions

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 86841cb..886e7e6 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -45,7 +45,9 @@
 #include <initializer_list>
 #endif
 
-#if __cplusplus >= 201703L
+#include <bits/version.h>
+
+#ifdef __glibcxx_string_view // >= C++17
 # include <string_view>
 #endif
 
@@ -53,7 +55,6 @@
 # include <charconv>
 #endif
 
-#include <bits/version.h>
 
 #if ! _GLIBCXX_USE_CXX11_ABI
 # include "cow_string.h"
@@ -146,7 +147,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	return __p;
       }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       // A helper type for avoiding boiler-plate.
       typedef basic_string_view<_CharT, _Traits> __sv_type;
 
@@ -788,7 +789,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 #endif
 	}
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Construct string from a substring of a string_view.
        *  @param  __t   Source object convertible to string view.
@@ -944,7 +945,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       }
 #endif // C++11
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Set value to string constructed from a string_view.
        *  @param  __svt  An object convertible to string_view.
@@ -1439,7 +1440,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       { return this->append(__l.begin(), __l.size()); }
 #endif // C++11
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Append a string_view.
        *  @param __svt  An object convertible to string_view to be appended.
@@ -1556,7 +1557,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
         append(_InputIterator __first, _InputIterator __last)
         { return this->replace(end(), end(), __first, __last); }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view
       /**
        *  @brief  Append a string_view.
        *  @param __svt  An object convertible to string_view to be appended.
@@ -1809,7 +1810,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       }
 #endif // C++11
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Set value from a string_view.
        *  @param __svt  The source object convertible to string_view.
@@ -2090,7 +2091,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	return iterator(_M_data() + __pos);
       }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Insert a string_view.
        *  @param __pos  Position in string to insert at.
@@ -2542,7 +2543,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       { return this->replace(__i1, __i2, __l.begin(), __l.size()); }
 #endif // C++11
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Replace range of characters with string_view.
        *  @param __pos  The position to replace at.
@@ -2741,7 +2742,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       _GLIBCXX_NOEXCEPT
       { return this->find(__str.data(), __pos, __str.size()); }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Find position of a string_view.
        *  @param __svt  The object convertible to string_view to locate.
@@ -2807,7 +2808,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       _GLIBCXX_NOEXCEPT
       { return this->rfind(__str.data(), __pos, __str.size()); }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Find last position of a string_view.
        *  @param __svt  The object convertible to string_view to locate.
@@ -2891,7 +2892,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       _GLIBCXX_NOEXCEPT
       { return this->find_first_of(__str.data(), __pos, __str.size()); }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Find position of a character of a string_view.
        *  @param __svt  An object convertible to string_view containing
@@ -2980,7 +2981,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       _GLIBCXX_NOEXCEPT
       { return this->find_last_of(__str.data(), __pos, __str.size()); }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Find last position of a character of string.
        *  @param __svt  An object convertible to string_view containing
@@ -3068,7 +3069,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       _GLIBCXX_NOEXCEPT
       { return this->find_first_not_of(__str.data(), __pos, __str.size()); }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Find position of a character not in a string_view.
        *  @param __svt  A object convertible to string_view containing
@@ -3155,7 +3156,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       _GLIBCXX_NOEXCEPT
       { return this->find_last_not_of(__str.data(), __pos, __str.size()); }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Find last position of a character not in a string_view.
        *  @param __svt  An object convertible to string_view containing
@@ -3271,7 +3272,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	return __r;
       }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Compare to a string_view.
        *  @param __svt An object convertible to string_view to compare against.
@@ -4605,7 +4606,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
   constexpr
 #endif
   inline wstring
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
   __to_wstring_numeric(string_view __s)
 #else
   __to_wstring_numeric(const string& __s)
@@ -4808,7 +4809,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   } // inline namespace literals
 #endif // __glibcxx_string_udls
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_variant // >= C++17
   namespace __detail::__variant
   {
     template<typename> struct _Never_valueless_alt; // see <variant>
diff --git a/libstdc++-v3/include/bits/cow_string.h b/libstdc++-v3/include/bits/cow_string.h
index 740e046..d5b3979 100644
--- a/libstdc++-v3/include/bits/cow_string.h
+++ b/libstdc++-v3/include/bits/cow_string.h
@@ -467,7 +467,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _S_empty_rep() _GLIBCXX_NOEXCEPT
       { return _Rep::_S_empty_rep(); }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       // A helper type for avoiding boiler-plate.
       typedef basic_string_view<_CharT, _Traits> __sv_type;
 
@@ -685,7 +685,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	: _M_dataplus(_S_construct(__beg, __end, __a), __a)
 	{ }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Construct string from a substring of a string_view.
        *  @param  __t   Source object convertible to string view.
@@ -775,7 +775,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
 #endif // C++11
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Set value to string constructed from a string_view.
        *  @param  __svt An object convertible to  string_view.
@@ -1246,7 +1246,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { return this->append(__l.begin(), __l.size()); }
 #endif // C++11
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Append a string_view.
        *  @param __svt The object convertible to string_view to be appended.
@@ -1338,7 +1338,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	append(_InputIterator __first, _InputIterator __last)
 	{ return this->replace(_M_iend(), _M_iend(), __first, __last); }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Append a string_view.
        *  @param __svt The object convertible to string_view to be appended.
@@ -1496,7 +1496,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { return this->assign(__l.begin(), __l.size()); }
 #endif // C++11
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Set value from a string_view.
        *  @param __svt The source object convertible to string_view.
@@ -1703,7 +1703,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return iterator(_M_data() + __pos);
       }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Insert a string_view.
        *  @param __pos  Position in string to insert at.
@@ -2092,7 +2092,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { return this->replace(__i1, __i2, __l.begin(), __l.end()); }
 #endif // C++11
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Replace range of characters with string_view.
        *  @param __pos  The position to replace at.
@@ -2354,7 +2354,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       size_type
       find(_CharT __c, size_type __pos = 0) const _GLIBCXX_NOEXCEPT;
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Find position of a string_view.
        *  @param __svt  The object convertible to string_view to locate.
@@ -2432,7 +2432,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       size_type
       rfind(_CharT __c, size_type __pos = npos) const _GLIBCXX_NOEXCEPT;
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Find last position of a string_view.
        *  @param __svt  The object convertible to string_view to locate.
@@ -2515,7 +2515,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       find_first_of(_CharT __c, size_type __pos = 0) const _GLIBCXX_NOEXCEPT
       { return this->find(__c, __pos); }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Find position of a character of a string_view.
        *  @param __svt  An object convertible to string_view containing
@@ -2599,7 +2599,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       find_last_of(_CharT __c, size_type __pos = npos) const _GLIBCXX_NOEXCEPT
       { return this->rfind(__c, __pos); }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Find last position of a character of string.
        *  @param __svt  An object convertible to string_view containing
@@ -2680,7 +2680,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       find_first_not_of(_CharT __c, size_type __pos = 0) const
       _GLIBCXX_NOEXCEPT;
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Find position of a character not in a string_view.
        *  @param __svt  An object convertible to string_view containing
@@ -2762,7 +2762,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       find_last_not_of(_CharT __c, size_type __pos = npos) const
       _GLIBCXX_NOEXCEPT;
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Find last position of a character not in a string_view.
        *  @param __svt  An object convertible to string_view containing
@@ -2824,7 +2824,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return __r;
       }
 
-#if __cplusplus >= 201703L
+#ifdef __glibcxx_string_view // >= C++17
       /**
        *  @brief  Compare to a string_view.
        *  @param __svt An object convertible to string_view to compare against.