libstdc++: Fix tests for constexpr std::string

Some tests fail when run with -D_GLIBCXX_USE_CXX11_ABI or -stdgnu++20.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (operator<=>): Use constexpr
	unconditionally.
	* testsuite/21_strings/basic_string/modifiers/constexpr.cc:
	Require cxx11-abit effective target.
	* testsuite/21_strings/headers/string/synopsis.cc: Add
	conditional constexpr to declarations, and adjust relational
	operators for C++20.

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

@@ -3546,8 +3546,7 @@ _GLIBCXX_END_NAMESPACE_CXX11
    *	       greater than, or incomparable with `__rhs`.
    */
   template<typename _CharT, typename _Traits, typename _Alloc>
-    _GLIBCXX20_CONSTEXPR
+    constexpr auto
-    inline auto
     operator<=>(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
 		const basic_string<_CharT, _Traits, _Alloc>& __rhs) noexcept
     -> decltype(__detail::__char_traits_cmp_cat<_Traits>(0))
@@ -3561,8 +3560,7 @@ _GLIBCXX_END_NAMESPACE_CXX11
    *	       greater than, or incomparable with `__rhs`.
    */
   template<typename _CharT, typename _Traits, typename _Alloc>
-    _GLIBCXX20_CONSTEXPR
+    constexpr auto
-    inline auto
     operator<=>(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
 		const _CharT* __rhs) noexcept
     -> decltype(__detail::__char_traits_cmp_cat<_Traits>(0))

libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc

@@ -1,5 +1,6 @@
 // { dg-options "-std=gnu++20" }
 // { dg-do compile { target c++20 } }
+// { dg-require-effective-target cxx11-abi }
 
 #include <string>
 #include <testsuite_hooks.h>

libstdc++-v3/testsuite/21_strings/headers/string/synopsis.cc

@@ -26,6 +26,12 @@
 # define NOTHROW
 #endif
 
+#if __cplusplus >= 202002L
+# define CONSTEXPR constexpr
+#else
+# define CONSTEXPR
+#endif
+
 namespace std {
   //  lib.char.traits, character traits:
   template<class charT>
@@ -40,33 +46,52 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 _GLIBCXX_END_NAMESPACE_CXX11
 
   template<class charT, class traits, class Allocator>
+  CONSTEXPR
   basic_string<charT,traits,Allocator>
   operator+(const basic_string<charT,traits,Allocator>& lhs,
 	    const basic_string<charT,traits,Allocator>& rhs);
   template<class charT, class traits, class Allocator>
+  CONSTEXPR
   basic_string<charT,traits,Allocator>
   operator+(const charT* lhs,
 	    const basic_string<charT,traits,Allocator>& rhs);
   template<class charT, class traits, class Allocator>
+  CONSTEXPR
   basic_string<charT,traits,Allocator>
   operator+(charT lhs, const basic_string<charT,traits,Allocator>& rhs);
   template<class charT, class traits, class Allocator>
+  CONSTEXPR
   basic_string<charT,traits,Allocator>
   operator+(const basic_string<charT,traits,Allocator>& lhs,
 	    const charT* rhs);
   template<class charT, class traits, class Allocator>
+  CONSTEXPR
   basic_string<charT,traits,Allocator>
   operator+(const basic_string<charT,traits,Allocator>& lhs, charT rhs);
 
   template<class charT, class traits, class Allocator>
+  CONSTEXPR
   bool operator==(const basic_string<charT,traits,Allocator>& lhs,
 		  const basic_string<charT,traits,Allocator>& rhs) NOTHROW;
   template<class charT, class traits, class Allocator>
-  bool operator==(const charT* lhs,
+  CONSTEXPR
-		  const basic_string<charT,traits,Allocator>& rhs);
-  template<class charT, class traits, class Allocator>
   bool operator==(const basic_string<charT,traits,Allocator>& lhs,
 		  const charT* rhs);
+
+#if __cpp_lib_three_way_comparison
+  template<class charT, class traits, class Allocator>
+  constexpr
+  bool operator<=>(const basic_string<charT,traits,Allocator>& lhs,
+		   const basic_string<charT,traits,Allocator>& rhs) NOTHROW;
+  template<class charT, class traits, class Allocator>
+  constexpr
+  bool operator<=>(const basic_string<charT,traits,Allocator>& lhs,
+		   const charT* rhs);
+#else
+  template<class charT, class traits, class Allocator>
+  CONSTEXPR
+  bool operator==(const charT* lhs,
+		  const basic_string<charT,traits,Allocator>& rhs);
   template<class charT, class traits, class Allocator>
   bool operator!=(const basic_string<charT,traits,Allocator>& lhs,
 		  const basic_string<charT,traits,Allocator>& rhs) NOTHROW;
@@ -114,9 +139,11 @@ _GLIBCXX_END_NAMESPACE_CXX11
   template<class charT, class traits, class Allocator>
   bool operator>=(const charT* lhs,
 		  const basic_string<charT,traits,Allocator>& rhs);
+#endif
 
   //  lib.string.special:
   template<class charT, class traits, class Allocator>
+  CONSTEXPR
   void swap(basic_string<charT,traits,Allocator>& lhs,
 	    basic_string<charT,traits,Allocator>& rhs)
 #if __cplusplus >= 201103L