From ba7cd28513049c9e1dd05493c73314f67b37c831 Mon Sep 17 00:00:00 2001
From: Adam Panzica <adampanzica@gmail.com>
Date: Thu, 2 Jul 2026 15:40:46 -0400
Subject: [PATCH] Avoid clang 21/22 C++26-mode frontend crash in StringLiteral
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Clang 21.1+, 22.1.x, and current trunk crash (SIGSEGV in
CheckParameterPacksForExpansion; assertion builds fail in
LocalInstantiationScope::findInstantiationOf) when compiling any
translation unit that includes reflect-cpp headers at -std=c++26,
because StringLiteral's variadic constructor constrains itself with a
requires-fold over decltype of its own parameter pack — a pattern that
hits an open clang regression from the CWG2369 reapplication
(llvm/llvm-project#198052; reported against reflect-cpp specifically in
llvm/llvm-project#205000, where a minimized reproducer is attached).

Constraining via template type parameters instead is semantically
equivalent (each argument must still be exactly char) and compiles
cleanly at -std=c++20/23/26 on all clang versions tested, including the
affected ones.

Authored with an AI assistant (Claude Code), reviewed and submitted by
the account owner.
---
 include/rfl/internal/StringLiteral.hpp | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/include/rfl/internal/StringLiteral.hpp b/include/rfl/internal/StringLiteral.hpp
index 03448d4e0..3c59036ac 100644
--- a/include/rfl/internal/StringLiteral.hpp
+++ b/include/rfl/internal/StringLiteral.hpp
@@ -3,6 +3,7 @@
 
 #include <algorithm>
 #include <array>
+#include <concepts>
 #include <string>
 #include <string_view>
 
@@ -14,9 +15,11 @@ namespace internal {
 /// for the parameters names in the NamedTuples.
 template <size_t N>
 struct StringLiteral {
-  constexpr StringLiteral(const auto... _chars)
-    requires (std::is_same_v<decltype(_chars), const char> && ...)
-    : arr_{_chars..., '\0'} {}
+  /// Constrained via template type parameters rather than a requires-fold
+  /// over decltype of the function parameter pack: the latter crashes
+  /// clang 21/22 in C++26 mode (llvm/llvm-project#198052, #205000).
+  template <std::same_as<char>... Chars>
+  constexpr StringLiteral(const Chars... _chars) : arr_{_chars..., '\0'} {}
 
   constexpr StringLiteral(const std::array<char, N> _arr) : arr_(_arr) {}