From 8b72fcd0d2e02725fa1a58b265afa3485568be09 Mon Sep 17 00:00:00 2001
From: Ivan Lazaric <ivan.lazaric1@gmail.com>
Date: Sun, 20 Sep 2026 23:21:52 +0200
Subject: [PATCH] Make `format_custom()` constexpr to improve constexpr
 formatting

Added test shows a simple example when this matters.
---
 include/fmt/core.h   |  5 +++--
 test/compile-test.cc | 24 ++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index f0815206..4dff1da0 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -2275,8 +2275,9 @@ template <typename Context> class value {
 
   // Formats an argument of a custom type, such as a user-defined class.
   template <typename T>
-  static void format_custom(void* arg, parse_context<char_type>& parse_ctx,
-                            Context& ctx) {
+  static FMT_CONSTEXPR void format_custom(void* arg,
+                                          parse_context<char_type>& parse_ctx,
+                                          Context& ctx) {
     auto f = formatter<T, char_type>();
     parse_ctx.advance_to(f.parse(parse_ctx));
     using qualified_type =
diff --git a/test/compile-test.cc b/test/compile-test.cc
index c3331273..77a7bce2 100644
--- a/test/compile-test.cc
+++ b/test/compile-test.cc
@@ -494,3 +494,27 @@ TEST(compile_test, format_as) {
   // compiled field path rather than the to_string fast path.
   EXPECT_EQ("[42]", fmt::format(FMT_COMPILE("[{}]"), type_with_format_as{42}));
 }
+
+#if FMT_CPLUSPLUS >= 202000L
+struct consteval_printable {};
+
+template<>
+struct fmt::formatter<consteval_printable> {
+  bool debug = false;
+  consteval auto parse(auto& ctx) {
+    if (ctx.begin() != ctx.end() && *ctx.begin() == '?') {
+      debug = true;
+      ctx.advance_to(ctx.begin() + 1);
+    }
+    return ctx.begin();
+  }
+  consteval auto format(consteval_printable, auto& ctx) const {
+    return fmt::formatter<std::string_view>{}.format(debug ? "debug" : "regular", ctx);
+  }
+};
+
+static_assert(fmt::format(FMT_COMPILE("{}"), consteval_printable{}) == "regular");
+static_assert(fmt::format(FMT_COMPILE("[{}]"), consteval_printable{}) == "[regular]");
+static_assert(fmt::format(FMT_COMPILE("{:?}"), consteval_printable{}) == "debug");
+static_assert(fmt::format(FMT_COMPILE("[{:?}]"), consteval_printable{}) == "[debug]");
+#endif // FMT_CPLUSPLUS >= 202000L