From de4705f84d16324c9496d0833e09656bc5d0e5f4 Mon Sep 17 00:00:00 2001
From: Barry Revzin <barry.revzin@gmail.com>
Date: Sat, 1 Jul 2023 08:46:04 -0500
Subject: [PATCH] Trying to improve errors in the unformattable case (#3478)

---
 include/fmt/core.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index 2323585b..ae80287e 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -1548,6 +1548,11 @@ constexpr auto encode_types() -> unsigned long long {
          (encode_types<Context, Args...>() << packed_arg_bits);
 }
 
+#if defined(__cpp_if_constexpr)
+// This type is intentionally undefined, only used for errors
+template <typename T, typename Char> struct type_is_unformattable_for;
+#endif
+
 template <bool PACKED, typename Context, typename T, FMT_ENABLE_IF(PACKED)>
 FMT_CONSTEXPR FMT_INLINE auto make_arg(T& val) -> value<Context> {
   using arg_type = remove_cvref_t<decltype(arg_mapper<Context>().map(val))>;
@@ -1565,6 +1570,11 @@ FMT_CONSTEXPR FMT_INLINE auto make_arg(T& val) -> value<Context> {
                 "Formatting of non-void pointers is disallowed.");
 
   constexpr bool formattable = !std::is_same<arg_type, unformattable>::value;
+#if defined(__cpp_if_constexpr)
+  if constexpr (!formattable) {
+    type_is_unformattable_for<T, typename Context::char_type> _;
+  }
+#endif
   static_assert(
       formattable,
       "Cannot format an argument. To make type T formattable provide a "
@@ -2529,7 +2539,17 @@ FMT_CONSTEXPR auto parse_format_specs(ParseContext& ctx)
       mapped_type_constant<T, context>::value != type::custom_type,
       decltype(arg_mapper<context>().map(std::declval<const T&>())),
       typename strip_named_arg<T>::type>;
+#if defined(__cpp_if_constexpr)
+  if constexpr (std::is_default_constructible_v<
+                    formatter<mapped_type, char_type>>) {
+    return formatter<mapped_type, char_type>().parse(ctx);
+  } else {
+    type_is_unformattable_for<T, char_type> _;
+    return ctx.begin();
+  }
+#else
   return formatter<mapped_type, char_type>().parse(ctx);
+#endif
 }
 
 // Checks char specs and returns true iff the presentation type is char-like.