[PATCH] c++: add feature-test macro for auto(x)

[Patrick Palka via Gcc-patches]

This adds the feature-test macro for PR0849R8, as per
https://github.com/cplusplus/CWG/issues/281.

Tested on x86_64-pc-linux-gnu, does this look OK for trunk/13?

gcc/c-family/ChangeLog:

        * c-cppbuiltin.cc (c_cpp_builtins): Predefine __cpp_auto_cast
        for C++23.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp23/feat-cxx2b.C: Test __cpp_auto_cast.
---
 gcc/c-family/c-cppbuiltin.cc            | 1 +
 gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C | 6 ++++++
 2 files changed, 7 insertions(+)

diff --git a/gcc/c-family/c-cppbuiltin.cc b/gcc/c-family/c-cppbuiltin.cc
index 98f5aef2af9..5d64625fcd7 100644
--- a/gcc/c-family/c-cppbuiltin.cc
+++ b/gcc/c-family/c-cppbuiltin.cc
@@ -1074,6 +1074,7 @@ c_cpp_builtins (cpp_reader *pfile)
          /* Set feature test macros for C++23.  */
          cpp_define (pfile, "__cpp_size_t_suffix=202011L");
          cpp_define (pfile, "__cpp_if_consteval=202106L");
+         cpp_define (pfile, "__cpp_auto_cast=202110L");
          cpp_define (pfile, "__cpp_constexpr=202211L");
          cpp_define (pfile, "__cpp_multidimensional_subscript=202211L");
          cpp_define (pfile, "__cpp_named_character_escapes=202207L");
diff --git a/gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C 
b/gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C
index 6f4f6bcaad0..9e29b01adc1 100644
--- a/gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C
+++ b/gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C
@@ -578,6 +578,12 @@
 #  error "__cpp_implicit_move != 202207"
 #endif
 
+#ifndef __cpp_auto_cast
+#  error "__cpp_auto_cast"
+#elif __cpp_auto_cast != 202110
+#  error "__cpp_auto_cast != 202110"
+#endif
+
 //  C++23 attributes:
 
 #ifdef __has_cpp_attribute
-- 
2.40.1.552.g91428f078b