From b5bd6323fcbe401e1fdda4ad550d87389d6a1efa Mon Sep 17 00:00:00 2001
From: Barry Revzin <barry.revzin@gmail.com>
Date: Sun, 4 Jun 2023 12:28:00 -0500
Subject: [PATCH] Trying to improve errors in the unformattable case.

---
 include/fmt/core.h | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index 4961310a..86dcd7a3 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -1539,12 +1539,18 @@ constexpr auto encode_types() -> unsigned long long {
          (encode_types<Context, Args...>() << packed_arg_bits);
 }
 
+// This type is intentionally undefined, only used for errors
+template <typename T, typename Char> struct type_is_unformattable_for;
+
 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))>;
 
   constexpr bool formattable_char =
       !std::is_same<arg_type, unformattable_char>::value;
+  if constexpr (not formattable_char) {
+    type_is_unformattable_for<T, typename Context::char_type> _;
+  }
   static_assert(formattable_char, "Mixing character types is disallowed.");
 
   // Formatting of arbitrary pointers is disallowed. If you want to format a
@@ -1552,10 +1558,16 @@ FMT_CONSTEXPR FMT_INLINE auto make_arg(T& val) -> value<Context> {
   // formatting of `[const] volatile char*` printed as bool by iostreams.
   constexpr bool formattable_pointer =
       !std::is_same<arg_type, unformattable_pointer>::value;
+  if constexpr (not formattable_pointer) {
+    type_is_unformattable_for<T, typename Context::char_type> _;
+  }
   static_assert(formattable_pointer,
                 "Formatting of non-void pointers is disallowed.");
 
   constexpr bool formattable = !std::is_same<arg_type, unformattable>::value;
+  if constexpr (not formattable) {
+    type_is_unformattable_for<T, typename Context::char_type> _;
+  }
   static_assert(
       formattable,
       "Cannot format an argument. To make type T formattable provide a "
@@ -2520,7 +2532,13 @@ 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>;
-  return formatter<mapped_type, char_type>().parse(ctx);
+  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();
+  }
 }
 
 // Checks char specs and returns true iff the presentation type is char-like.