From 601d546b8452159c38d65231d52f0dbd05e793d2 Mon Sep 17 00:00:00 2001
From: Michael Thies <mail@mhthies.de>
Date: Sun, 27 Aug 2023 18:58:03 +0200
Subject: Add explicit fmt::formatter for nlohmann::basic_json

The formatter is based on fmt's ostream_formatter, using the provided
operator<<(std::ostream&) of nlohmann::basic_json. This is required to
allow compilation against fmt 10.x

Co-authored-by: Oliver Reiche <oliver.reiche@huawei.com>
---
 src/utils/cpp/TARGETS  | 7 ++++++-
 src/utils/cpp/json.hpp | 7 +++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

(limited to 'src/utils/cpp')

diff --git a/src/utils/cpp/TARGETS b/src/utils/cpp/TARGETS
index 73c5e462..7c05472f 100644
--- a/src/utils/cpp/TARGETS
+++ b/src/utils/cpp/TARGETS
@@ -16,7 +16,12 @@
   { "type": ["@", "rules", "CC", "library"]
   , "name": ["json"]
   , "hdrs": ["json.hpp"]
-  , "deps": ["gsl", ["@", "json", "", "json"], ["@", "gsl", "", "gsl"]]
+  , "deps":
+    [ "gsl"
+    , ["@", "json", "", "json"]
+    , ["@", "gsl", "", "gsl"]
+    , ["@", "fmt", "", "fmt"]
+    ]
   , "stage": ["src", "utils", "cpp"]
   }
 , "concepts":
diff --git a/src/utils/cpp/json.hpp b/src/utils/cpp/json.hpp
index aed2f3f4..9fd2d66d 100644
--- a/src/utils/cpp/json.hpp
+++ b/src/utils/cpp/json.hpp
@@ -21,6 +21,7 @@
 #include <string>
 #include <unordered_map>
 
+#include "fmt/ostream.h"
 #include "gsl/gsl"
 #include "nlohmann/json.hpp"
 #include "src/utils/cpp/gsl.hpp"
@@ -212,4 +213,10 @@ namespace detail {
     return old_abbrev;
 }
 
+// Use nlohmann::basic_json::operator<<() for formatting via libfmt.
+// This explicit template specialization seems to be required starting with
+// libfmt 10.x.
+template <>
+struct fmt::formatter<nlohmann::basic_json<>> : ostream_formatter {};
+
 #endif  // INCLUDED_SRC_UTILS_CPP_JSON_HPP
-- 
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