From c07e2aa4b130b4fd359a1c6455a5201b17af01fa Mon Sep 17 00:00:00 2001
From: Jacek Caban <cjacek@gmail.com>
Date: Mon, 7 Sep 2026 16:13:51 +0200
Subject: [PATCH] Disable std::exception_ptr and nested_exception formatters
 when exceptions are disabled. (#4904)

Fixes building with Clang 20 and older with -fno-exceptions, which treats
try/catch usage as an error.
---
 include/fmt/std.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/fmt/std.h b/include/fmt/std.h
index 28a7f6c9..ed169de5 100644
--- a/include/fmt/std.h
+++ b/include/fmt/std.h
@@ -665,7 +665,7 @@ struct formatter<
     }
 #endif  // FMT_USE_RTTI
     out = detail::write_bytes<char>(out, string_view(ex.what()));
-#if FMT_USE_RTTI
+#if FMT_USE_RTTI && FMT_USE_EXCEPTIONS
     // If the exception carries a nested exception (e.g. via
     // std::throw_with_nested), format the whole chain.
     if (auto* nested = dynamic_cast<const std::nested_exception*>(&ex)) {
@@ -680,7 +680,7 @@ struct formatter<
         }
       }
     }
-#endif  // FMT_USE_RTTI
+#endif  // FMT_USE_RTTI && FMT_USE_EXCEPTIONS
     return out;
   }
 };
@@ -690,6 +690,7 @@ template <> struct formatter<std::exception_ptr> : formatter<std::exception> {
   auto format(const std::exception_ptr& ep, FormatContext& ctx) const
       -> decltype(ctx.out()) {
     if (!ep) return this->write_padded(ctx, string_view("none"));
+#if FMT_USE_EXCEPTIONS
     try {
       std::rethrow_exception(ep);
     } catch (const std::exception& e) {
@@ -697,6 +698,9 @@ template <> struct formatter<std::exception_ptr> : formatter<std::exception> {
     } catch (...) {
       return this->write_padded(ctx, string_view("unknown exception"));
     }
+#else
+    return this->write_padded(ctx, string_view("unknown exception"));
+#endif  // FMT_USE_EXCEPTIONS
   }
 };