libstdc++: Define __cpp_lib_three_way_comparison conditionally

The contents of the <compare> header are not complete unless concepts
are supported, so the feature test macro should depend on the macro for
concepts.

As a result, the std::lexicographical_compare_three_way function will
not be defined unless concepts are supported, so there is no need to
check __cpp_lib_concepts before using concepts in those functions.

	* include/bits/stl_algobase.h (__is_byte_iter, __min_cmp)
	(lexicographical_compare_three_way): Do not depend on
	__cpp_lib_concepts.
	* include/std/version (__cpp_lib_three_way_comparison): Only define
	when __cpp_lib_concepts is defined.
	* libsupc++/compare (__cpp_lib_three_way_comparison): Likewise.

From-SVN: r279896

libstdc++-v3/ChangeLog

+2020-01-06  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/bits/stl_algobase.h (__is_byte_iter, __min_cmp)
+	(lexicographical_compare_three_way): Do not depend on
+	__cpp_lib_concepts.
+	* include/std/version (__cpp_lib_three_way_comparison): Only define
+	when __cpp_lib_concepts is defined.
+	* libsupc++/compare (__cpp_lib_three_way_comparison): Likewise.
+
 2020-01-03  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/bits/stl_algobase.h (lexicographical_compare_three_way):

libstdc++-v3/include/bits/stl_algobase.h

@@ -1667,7 +1667,6 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
     }
 
 #if __cpp_lib_three_way_comparison
-#if __cpp_lib_concepts
   // Iter points to a contiguous range of unsigned narrow character type
   // or std::byte, suitable for comparison by memcmp.
   template<typename _Iter>
@@ -1690,7 +1689,6 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
 	return _Res{__y, __c};
       return _Res{__x, __c};
     }
-#endif
 
   /**
    *  @brief Performs dictionary comparison on ranges.
@@ -1718,7 +1716,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
       __glibcxx_requires_valid_range(__first1, __last1);
       __glibcxx_requires_valid_range(__first2, __last2);
 
-#if __cpp_lib_concepts && __cpp_lib_is_constant_evaluated
+#if __cpp_lib_is_constant_evaluated
       using _Cat = decltype(__comp(*__first1, *__first2));
       static_assert(same_as<common_comparison_category_t<_Cat>, _Cat>);
 
@@ -1739,7 +1737,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
 		  }
 		return __lencmp;
 	      }
-#endif // concepts && is_constant_evaluated
+#endif // is_constant_evaluated
       while (__first1 != __last1 && __first2 != __last2)
 	{
 	  if (auto __cmp = __comp(*__first1, *__first2); __cmp != 0)
@@ -1751,7 +1749,6 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
 	: __first2 != __last2 ? strong_ordering::less : strong_ordering::equal;
     }
 
-#if __cpp_lib_concepts
   template<typename _InputIter1, typename _InputIter2>
     constexpr auto
     lexicographical_compare_three_way(_InputIter1 __first1,
@@ -1763,7 +1760,6 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
 						    __first2, __last2,
 						    compare_three_way{});
     }
-#endif // concepts
 #endif // three_way_comparison
 
   template<typename _InputIterator1, typename _InputIterator2,

libstdc++-v3/include/std/version

@@ -191,7 +191,7 @@
 #define __cpp_lib_list_remove_return_type 201806L
 #define __cpp_lib_math_constants 201907L
 #define __cpp_lib_span 201902L
-#if __cpp_impl_three_way_comparison >= 201907L
+#if __cpp_impl_three_way_comparison >= 201907L && __cpp_lib_concepts
 # define __cpp_lib_three_way_comparison 201711L
 #endif
 #define __cpp_lib_to_array 201907L

libstdc++-v3/libsupc++/compare

@@ -38,10 +38,12 @@
 
 #include <concepts>
 
+#if __cpp_lib_concepts
+# define __cpp_lib_three_way_comparison 201711L
+#endif
+
 namespace std
 {
-#define __cpp_lib_three_way_comparison 201711L
-
   // [cmp.categories], comparison category types
 
   namespace __cmp_cat