Compilation with clang likely broken?

[MortenMacFly]

I am trying to compile an App that should make use of json.hpp and it works fine for ages with GCC compilers.

Now I am in the progress of switching to clang (on Windows) from here: https://github.com/mstorsjo/llvm-mingw/releases
Since then I am getting this error:

 21839 |     ValueType & get_to(ValueType& v) const noexcept(noexcept(
 21840 |             JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>(), v)))
       |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 21841 |     {
       |     ~
 21842 |         JSONSerializer<ValueType>::from_json(*this, v);
       |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 21843 |         return v;
       |         ~~~~~~~~~
 21844 |     }
       |     ~
protocol.h:735:61: note: while substituting deduced template arguments into function template 'get_to' [with ValueType = std::vector<Diagnostic>, $1 = (no value)]
  735 | JSON_SERIALIZE(PublishDiagnosticsParams, {}, {FROM_KEY(uri);FROM_KEY(diagnostics);});
protocol.h:23:56: note: expanded from macro 'FROM_KEY'
   23 | #define FROM_KEY(KEY) if (j.contains(#KEY)) j.at(#KEY).get_to(value.KEY);
protocol.h:38:83: note: expanded from macro 'JSON_SERIALIZE'
   38 |             static void from_json(cb_unused const json& j, cb_unused Type& value) FROM \
protocol.h:687:8: note: forward declaration of 'CodeAction'
  687 | struct CodeAction;
In file included from codecompletion.cpp:15:
In file included from C:/MinGW64_LLVMARM/include/c++/v1/set:1547:
In file included from C:/MinGW64_LLVMARM/include/c++/v1/functional:601:
In file included from C:/MinGW64_LLVMARM/include/c++/v1/vector:314:
C:/MinGW64_LLVMARM/include/c++/v1/__vector/vector.h:749:44: error: arithmetic on a pointer to an incomplete type 'const value_type' (aka 'const CodeAction')

And to me, thix looks correct as CodeAction at that line of use is not fully declared. However, it also cannot be because its a linkage between Diagnostics and CodeAction in the end, so the one or other must be undefined.

My question: Is this a limitation of clang or could/should this also be fixed in json.hpp itself?

[nlohmann]

This is very likely not a json.hpp bug but a difference in how libstdc++ (GCC) vs. libc++ (clang/llvm-mingw) handle incomplete types in std::vector.

get_to/from_json end up instantiating std::vector’s internals (including pointer arithmetic between elements) for T = CodeAction. That requires CodeAction to be a complete type at the point of instantiation — libstdc++ tends to tolerate this in more situations, while libc++ enforces it strictly (hence the arithmetic on a pointer to an incomplete type error).

Given the circular relationship between Diagnostics and CodeAction, the fix is on your side: make sure CodeAction is fully defined before Diagnostics’/PublishDiagnosticsParams’ from_json/get_to machinery is instantiated for the vector containing it (e.g. reorder declarations/includes), or break the cycle by storing CodeAction via std::unique_ptr instead of by value until it’s complete.

[MortenMacFly]

Thank you for the clarification. Hence, maybe you can help me with what you mean "fix is on my side"? Do you mean I should patch the json files? I am asking because its not my part of the code that triggers the error, maybe I get your hint wrong?

Other than that, I guess I will also report to the LLVM/MinGW team to see if they ca do anything about it...

[nlohmann]

Can you share the definitions of Diagnostics, PublishDiagnosticsParams, and CodeAction?

[MortenMacFly]

Can you share the definitions of Diagnostics, PublishDiagnosticsParams, and CodeAction?

The original issue comes from this code/file: Protocol.h (from here) which in return stems mostly from the clang sources. In line 687 there is the forward declaration of CodeAction, used in the structure Diagnostic following directly after. CodeAction is actually being fully defined in line 790. But it also has an option field <std::vector<Diagnostic>> of the Diagnostics type again and this raises the cyclic issue abut also does not allow me to re-order easily.

But I guess you are totally right, its probably not an issue with your code but rather an issue with either clang or libc++, maybe nothing to worry about yourself. If, however, you have an idea, let me know - but its likely out of scope for this place. (Feel free to close!)

[nlohmann]

I could reproduce this with the actual protocol.h from the Code::Blocks tree. It is not a json.hpp issue, and it is also not really a clang bug — it only shows up in C++20 mode with libc++:

• clang 21 + libc++, -std=c++17: compiles
• clang 21 + libc++, -std=c++20/-std=c++23: fails with exactly your diagnostic (same note trail: protocol.h:735 → FROM_KEY → JSON_SERIALIZE → forward declaration at protocol.h:687)
• GCC 16 + libstdc++: compiles in all modes

Same result with json 3.11.3 and 3.12.0, so the library version does not matter here.

The reason: Diagnostic (line 689) has option<std::vector<CodeAction>> while CodeAction is only forward-declared (line 687, defined at line 790). The serializer at line 735 instantiates std::vector<Diagnostic>'s operations, which require Diagnostic's special member functions, which require std::vector<CodeAction>'s, which require CodeAction to be complete. In C++17 libc++ can defer that instantiation to the end of the translation unit (where CodeAction is complete); since C++20 std::vector's members are constexpr and are instantiated eagerly at the point of use, so the incomplete type is diagnosed. std::vector<T> only allows an incomplete T up to the point where a member is referenced, so this is on thin ice with any implementation — libstdc++ is simply more forgiving.

The fix is a pure reordering in protocol.h — no change to the structs, and the Diagnostic ↔ CodeAction cycle stays as it is. Move these four JSON_SERIALIZE invocations so that they come after the definition of struct CodeAction (i.e. after line 806), keeping their relative order:

• lines 722–723 — Diagnostic
• line 735 — PublishDiagnosticsParams
• line 742 — CodeActionContext
• line 754 — CodeActionParams

The struct definitions themselves stay where they are. I verified that the patched header compiles with clang/libc++ and with GCC in C++17, C++20 and C++23.

(This reply was written by Claude Code.)

[MortenMacFly]

I am impressed: This really worked! Thank you very much, now the issue can really be closed.