Avoid clang 21/22 C++26-mode frontend crash in StringLiteral's variadic constructor

[aruisdante]

Problem

Any translation unit that includes reflect-cpp headers fails to compile at -std=c++26 on clang 21.1+, 22.1.x, and current trunk: the compiler itself crashes (SIGSEGV in CheckParameterPacksForExpansion; assertion builds fail with "declaration not instantiated in this scope" in LocalInstantiationScope::findInstantiationOf) while parsing using aliases to Literal<...> / NamedTuple<...> types — e.g. parsing/Parser_duration.hpp and parsing/Parser_result.hpp are the first two headers hit. -std=c++23 and earlier are unaffected.

Root cause is an open clang regression from the CWG2369 reapplication, not a reflect-cpp bug: llvm/llvm-project#198052, reported against reflect-cpp specifically in llvm/llvm-project#205000 (an 11-line minimized reproducer, reduced from rfl::internal::StringLiteral, is attached there). The trigger is StringLiteral's variadic constructor constraining itself with a requires-fold over decltype of its own function parameter pack, alongside the competing const char (&)[N] constructor.

Change

Constrain via template type parameters instead:

template <std::same_as<char>... Chars>
constexpr StringLiteral(const Chars... _chars) : arr_{_chars..., '\0'} {}

Semantically equivalent — each argument must still be exactly char — and it sidesteps the clang bug on every version tested. Since the crash fires at parse time, this one constructor is the only change needed to make the whole library usable at C++26 on affected clangs; it can be reverted freely once fixed clang releases are common.

Testing

Verified with clang 22.1.7 (libc++, -fsyntax-only and full builds): with this change, including rfl/json.hpp/rfl/cbor.hpp compiles cleanly at -std=c++20/23/26, and CBOR + JSON round-trips exercising Literal, Field, TaggedUnion, enums, optionals, and containers pass, including under ASan/UBSan. Please let your CI run the full suite to confirm.

Disclosure

This fix was authored with an AI assistant (Claude Code) and reviewed/submitted by the account owner.

[gemini-code-assist]

Code Review

This pull request updates StringLiteral.hpp to constrain the StringLiteral constructor using template type parameters (std::same_as<char>...) instead of a requires-fold over decltype of the function parameter pack. This change, along with the addition of the <concepts> header, prevents compiler crashes in Clang 21/22 when compiling in C++26 mode. There are no review comments, and I have no additional feedback to provide.

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