[gcc r12-5326] libstdc++: Fix tests for constexpr std::string

[Jonathan Wakely]

https://gcc.gnu.org/g:73e4d9f1751fd6d3c182988b2f9d0199c86053c9

commit r12-5326-g73e4d9f1751fd6d3c182988b2f9d0199c86053c9
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Nov 16 21:00:50 2021 +0000

    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.

Diff:
---
 libstdc++-v3/include/bits/basic_string.h           |  6 ++--
 .../21_strings/basic_string/modifiers/constexpr.cc |  1 +
 .../21_strings/headers/string/synopsis.cc          | 33 ++++++++++++++++++++--
 3 files changed, 33 insertions(+), 7 deletions(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index b6945f1cdfb..0b7d6c0a981 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/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
-    inline auto
+    constexpr 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
-    inline auto
+    constexpr auto
     operator<=>(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
 		const _CharT* __rhs) noexcept
     -> decltype(__detail::__char_traits_cmp_cat<_Traits>(0))
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc
index c875a3a19ad..a4627714d9a 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc
+++ b/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>
diff --git a/libstdc++-v3/testsuite/21_strings/headers/string/synopsis.cc b/libstdc++-v3/testsuite/21_strings/headers/string/synopsis.cc
index f14c4ae831c..f12345ed426 100644
--- a/libstdc++-v3/testsuite/21_strings/headers/string/synopsis.cc
+++ b/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,
-		  const basic_string<charT,traits,Allocator>& rhs);
-  template<class charT, class traits, class Allocator>
+  CONSTEXPR
   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