[Build] jsoncpp 1.8.4 compilation fails with clang-10

[bshastry]

Description

clang-10 (unreleased) reports a type conversion error here

https://github.com/open-source-parsers/jsoncpp/blob/ddabf50f72cf369bf652a95c4d9fe31a1865a781/src/lib_json/json_value.cpp#L70-L73

Likely root cause is https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es46-avoid-lossy-narrowing-truncating-arithmetic-conversions

[ekpyron]

Would be solved (with quite a big hammer) by #6900 ;-).

[bshastry]

This currently fails the fuzzing build. Working on a (temporary) fix.

[bshastry]

I have not been able to "hot" patch this so far. The closest I came is to remove the -Werror=conversion option here

https://github.com/open-source-parsers/jsoncpp/blob/2cf939e8c37494922ca2991902a0fe50baa2eaea/CMakeLists.txt#L127

Sadly, even I were to add -Wno-error=conversion here

solidity/cmake/jsoncpp.cmake

Line 22 in 2508cbc

set(JSONCPP_CXX_FLAGS ${CMAKE_CXX_FLAGS})

it will be undone by the linked cmake because jsoncpp's -Werror=conversion follows the options supplied by CMAKE_CXX_FLAGS (see first link)

[ekpyron]

What happens if you add -DJSON_USE_INT64_DOUBLE_CONVERSION to JSONCPP_CXX_FLAGS? Even if that works, we'd need to have a close look at what that actually does, though :-).

[bshastry]

Nice find :-)

Sadly, I cannot just pass -DJSON_USE... to JSONCPP_CXX_FLAGS because cmake-3.14 (version in the ossfuzz docker image) complains of definition not being of VAR=VALUE form, and when I do change it to the form it warns me that the passed option is not used by jsoncpp

Manually-specified variables were not used by the project:                                                                                   JSON_USE_INT64_DOUBLE_CONVERSION

[ekpyron]

@bshastry That sounds a bit weird... are you sure there aren't just some quotes missing which will make cmake consider it as a -D-style cmake option instead of a list of cxx flags?

[bshastry]

Yup, the problem was missing double quotes 🤦‍♂️