[clockblocker]

AI-authored

Explains the failure in #700 in the error message and in the docs. The read still fails.

struct Person {
  rfl::DefaultVal<std::string> name = "Homer Simpson";
  rfl::Validator<std::vector<std::string>, rfl::Size<rfl::Minimum<1>>> favorite_foods;
};

rfl::json::read<Person>(R"({"favorite_foods":["donuts","pizza"]})");

main:        Size validation failed: Value expected to be greater than or equal to 1, but got 0.
this branch: Could not default-construct Person, which is required because it contains rfl::DefaultVal. Give every field a valid default value. Original error: Size validation failed: Value expected to be greater than or equal to 1, but got 0.

For a struct without rfl::DefaultVal that is read with rfl::DefaultIfMissing, the reason reads because it is read with rfl::DefaultIfMissing.

What changes

• Error text: when T{} throws in read_struct_with_default, the error names the struct, says why it was default-constructed and what to do, and ends with the original message.
• Docs: a new section "All fields need a valid default value" in docs/content/default_val.md and a note under rfl::DefaultIfMissing in docs/content/processors.md. They show the workaround:

rfl::Validator<std::vector<std::string>, rfl::Size<rfl::Minimum<1>>> favorite_foods =
    std::vector<std::string>({"none"});

The field stays required (Field named 'favorite_foods' not found.). Only with rfl::DefaultIfMissing does the initializer become the value of a missing field.

What does not change

• No behaviour or API change: every read fails or succeeds as on main. Errors from reading the fields keep their text.
• Code that compares error().what() with the old text for this case no longer matches.

Tests: tests/json/test_default_construct_error.cpp, 6 tests, 4 of which fail on main. Run locally: 504 of 504 in ctest with Apple clang 17; json 284 of 284 and generic 99 of 99 with GCC 16.2. Not run: Linux, MSVC, older compilers, the other formats.

Related: #778's test expects the bare validator message, so it needs a one-line update when both are merged. #780 builds on this PR and makes the read succeed.

🤖 Generated with Claude Code

[liuzicheng1987]

@clockblocker thanks for the contribution. I am merging.