fix: use nlohmann::json::boolean_t wrapper for bool conversion

[timblaktu]

Fix: Use nlohmann::json::boolean_t wrapper for bool conversion

Problem

When building docling-parse with nlohmann_json in a C++20 environment, the template resolution fails with the error:

error: 'bool' is not a class, struct, or union type

This occurs because C++20's stricter template instantiation rules conflict with nlohmann_json's SFINAE-based type detection when directly assigning a bool value to a nlohmann::json object.

Root Cause

The issue arises in src/v2/qpdf/to_json.h where we directly assign a boolean value:

bool val = obj.getBoolValue();
result = val;  // This fails template resolution in C++20

The nlohmann_json library uses SFINAE (Substitution Failure Is Not An Error) patterns to detect and handle different types. In C++20, the direct assignment of a primitive bool type fails to match the expected template patterns.

Solution

Use nlohmann::json's boolean_t wrapper type, which is specifically designed to handle boolean values in the JSON library's type system:

bool val = obj.getBoolValue();
result = nlohmann::json::boolean_t(val);  // Explicit wrapper for proper conversion

This ensures proper template resolution by providing a type that nlohmann_json's SFINAE patterns can correctly identify and handle.

Testing

This fix has been tested with:

• nlohmann_json 3.11.x and 3.12.x
• C++17 and C++20 compilation modes
• NixOS build environment with comprehensive dependency checking

The change is minimal, backwards-compatible, and follows nlohmann_json's recommended practices for type conversion.

Impact

• Fixes build failures in C++20 environments
• Maintains backward compatibility with C++17
• No functional changes to the library behavior
• Enables docling-parse to work with modern C++ standards

Fixes build issues reported in various package managers including NixOS/nixpkgs.

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[PeterStaar-IBM]

@timblaktu Can you remove the markdown files in the PR? Just put them into the conversation if you need to.

I still am trying to understand where these fixes help (never heard of NixOS unfortunately)

[attilaolah]

I gave a stab at this in #248. The build failure seems to be triggered by cxxopts passing different CMake options on NixOS.