From c91f3a9c093c0dd6a9f589cacf8c62d94d6a7e7a Mon Sep 17 00:00:00 2001
From: Kefu Chai <kefu.chai@scylladb.com>
Date: Fri, 15 Mar 2024 10:10:01 +0800
Subject: [PATCH] util: add fmt::formatter for bool_class<Tag>

this change addresses the formatting with fmtlib v10, which stopped
creating the default-generated formatter even if FMT_DEPRECATED_OSTREAM
preprocess macro is defined. so to enable Seastar applications to
print bool_class<Tag> using {fmt} without defining the formatter
by themselves, we provide the formatter for this class in Seastar.

the operator<< for bool_class<Tag> is preserved for backward
compatibility. please note, {fmt} format a `bool` variable as "true" or
"false" based on its value, so we don't have to put "true" or "false"
explicitly as we do in the implementation of operator<<.

Refs https://github.com/scylladb/scylladb/issues/13245

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes #2144
---
 include/seastar/util/bool_class.hh | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/include/seastar/util/bool_class.hh b/include/seastar/util/bool_class.hh
index fd28078af91..3ba028e0ae3 100644
--- a/include/seastar/util/bool_class.hh
+++ b/include/seastar/util/bool_class.hh
@@ -22,6 +22,7 @@
 #pragma once
 
 #include <ostream>
+#include <fmt/core.h>
 
 namespace seastar {
 
@@ -101,3 +102,12 @@ const bool_class<Tag> bool_class<Tag>::no { false };
 /// @}
 
 }
+
+template<typename Tag>
+struct fmt::formatter<seastar::bool_class<Tag>> {
+    constexpr auto parse(format_parse_context& ctx) { return ctx.begin(); }
+    template <typename FormatContext>
+    auto format(seastar::bool_class<Tag> v, FormatContext& ctx) const{
+        return fmt::format_to(ctx.out(), "{}", bool(v));
+    }
+};