From a770009fcc295b3a6f5b314e4eafaaadc5580444 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <victor.zverovich@gmail.com>
Date: Fri, 13 Dec 2019 11:28:09 -0800
Subject: [PATCH] Improve error reporting

---
 include/fmt/core.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index 9fd8df28..f4fe1088 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -930,6 +930,16 @@ template <typename Context> struct arg_mapper {
     std::memcpy(val.data, &arg, sizeof(arg));
     return val;
   }
+
+  int map(...) {
+    constexpr bool formattable = sizeof(Context) == 0;
+    static_assert(
+        formattable,
+        "Cannot format argument. To make type T formattable provide a "
+        "formatter<T> specialization: "
+        "https://fmt.dev/latest/api.html#formatting-user-defined-types");
+    return 0;
+  }
 };
 
 // A type constant after applying arg_mapper<Context>.