[clang-tools-extra] [llvm] [compiler-rt] [mlir] [clang] [libc] [flang] [libcxx] [lldb] [lld] [libc++][format] P2637R3: Member `visit` (`std::basic_format_arg`) (PR #76449)

[Mark de Wever via cfe-commits]

================
@@ -163,20 +163,26 @@ class _LIBCPP_TEMPLATE_VIS basic_format_context<typename __format::__retarget_bu
 #  endif
         __ctx_(std::addressof(__ctx)),
         __arg_([](void* __c, size_t __id) {
-          return std::visit_format_arg(
-              [&](auto __arg) -> basic_format_arg<basic_format_context> {
-                if constexpr (same_as<decltype(__arg), monostate>)
-                  return {};
-                else if constexpr (same_as<decltype(__arg), typename basic_format_arg<_Context>::handle>)
-                  // At the moment it's not possible for formatting to use a re-targeted handle.
-                  // TODO FMT add this when support is needed.
-                  std::__throw_format_error("Re-targeting handle not supported");
-                else
-                  return basic_format_arg<basic_format_context>{
-                      __format::__determine_arg_t<basic_format_context, decltype(__arg)>(),
-                      __basic_format_arg_value<basic_format_context>(__arg)};
-              },
-              static_cast<_Context*>(__c)->arg(__id));
+          auto __visitor = [&](auto __arg) -> basic_format_arg<basic_format_context> {
+            if constexpr (same_as<decltype(__arg), monostate>)
+              return {};
+            else if constexpr (same_as<decltype(__arg), typename basic_format_arg<_Context>::handle>)
+              // At the moment it's not possible for formatting to use a re-targeted handle.
+              // TODO FMT add this when support is needed.
+              std::__throw_format_error("Re-targeting handle not supported");
+            else
+              return basic_format_arg<basic_format_context>{
+                  __format::__determine_arg_t<basic_format_context, decltype(__arg)>(),
+                  __basic_format_arg_value<basic_format_context>(__arg)};
+          };
+#  if _LIBCPP_STD_VER >= 26 && (!defined(_LIBCPP_COMPILER_CLANG_BASED) || _LIBCPP_CLANG_VER >= 1800)
+          return static_cast<_Context*>(__c)->arg(__id).visit(std::move(__visitor));
+#  else
+          _LIBCPP_DIAGNOSTIC_PUSH
+          _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wdeprecated-declarations")
+          return std::visit_format_arg(std::move(__visitor), static_cast<_Context*>(__c)->arg(__id));
+          _LIBCPP_DIAGNOSTIC_POP
----------------
mordante wrote:

```suggestion
          _LIBCPP_SUPPRESS_DEPRECATED_POP
```

https://github.com/llvm/llvm-project/pull/76449