[gcc r16-7153] libstdc++: Fix use of feature test macro in <source_location>

[Jonathan Wakely]

https://gcc.gnu.org/g:a111fd4d47243b7b9a9daabbc79fd4a2c7339dd4

commit r16-7153-ga111fd4d47243b7b9a9daabbc79fd4a2c7339dd4
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jan 29 17:02:43 2026 +0000

    libstdc++: Fix use of feature test macro in <source_location>
    
    libstdc++-v3/ChangeLog:
    
            * include/std/source_location: Check __glibcxx_contracts instead
            of __cpp_lib_contracts.
            * testsuite/18_support/contracts/includes.cc: New test.

Diff:
---
 libstdc++-v3/include/std/source_location                | 13 +++++--------
 libstdc++-v3/testsuite/18_support/contracts/includes.cc |  6 ++++++
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/libstdc++-v3/include/std/source_location b/libstdc++-v3/include/std/source_location
index 170753fdde2a..f773879755c9 100644
--- a/libstdc++-v3/include/std/source_location
+++ b/libstdc++-v3/include/std/source_location
@@ -38,12 +38,9 @@
 namespace std
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
-#ifdef __cpp_lib_contracts
-  namespace contracts
-  {
-    class contract_violation;
-  }
-#endif // __cpp_lib_contracts
+#ifdef __glibcxx_contracts
+  namespace contracts { class contract_violation; }
+#endif
 
   /// A class that describes a location in source code.
   struct source_location
@@ -95,10 +92,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     constexpr source_location (const void *__t)
       : _M_impl (static_cast <const __impl*>(__t)) {}
 
-#ifdef __cpp_lib_contracts
+#ifdef __glibcxx_contracts
     /* To enable use of the source __impl*.  */
     friend class std::contracts::contract_violation;
-#endif // __cpp_lib_contracts
+#endif
   };
 
 _GLIBCXX_END_NAMESPACE_VERSION
diff --git a/libstdc++-v3/testsuite/18_support/contracts/includes.cc b/libstdc++-v3/testsuite/18_support/contracts/includes.cc
new file mode 100644
index 000000000000..91f793f1e201
--- /dev/null
+++ b/libstdc++-v3/testsuite/18_support/contracts/includes.cc
@@ -0,0 +1,6 @@
+// { dg-options "-fcontracts" }
+// { dg-do compile { target c++26 } }
+
+// We should not get errors from including this before <contracts>:
+#include <source_location>
+#include <contracts>