Fix zero-member serialization macros

[dark-ous]

Summary

Fixes #4041 by allowing the NLOHMANN_DEFINE_TYPE_* serialization macros to be used with zero-member types.

Previously, invoking these macros on an empty type resulted in invalid macro expansion and compilation failed. With this change, zero-member types can be serialized and deserialized correctly as empty JSON objects ({}).

Example:

struct Empty
{
    NLOHMANN_DEFINE_TYPE_INTRUSIVE(Empty)
};

Changes

• Handle zero-member macro expansion in the serialization macros.
• Serialize zero-member types as empty JSON objects ({}).
• Preserve the existing behavior for types with member variables.
• Add regression tests covering zero-member intrusive and non-intrusive types.

Implementation

The implementation handles the empty variadic argument case by using ##__VA_ARGS__ to remove the trailing comma when no member arguments are supplied and introduces a no-op NLOHMANN_JSON_PASTE1 expansion for the zero-member case.

Testing

• Added regression tests for zero-member intrusive and non-intrusive serialization macros.
• Verified the original reproducer from NLOHMANN_DEFINE_TYPE_* fails with zero members #4041.
• Ran the full local test suite successfully (109/109 tests passed).

---

• The changes are described in detail, both the what and why.
• An existing issue is referenced (NLOHMANN_DEFINE_TYPE_* fails with zero members #4041).
• Code coverage remains at 100% with regression tests for the new behavior.
• Documentation update is not required.
• The source code has been amalgamated by running make amalgamate.

[github-actions]

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