Assert when using std::views::filter and GCC 10

[gwjo]

Description

After upgrading from 3.9.1 to 3.11.3 the code asserts when iterating over a json array that's been filtered using std::views::filter due to an uninitialized iterator.

In addition to my failure with 3.11.3 and GCC 10.2, I checked other combinations on Godbolt and observed the following (❌ represents the assert is seen)

	GCC 10.2	GCC 10.5	GCC 11.1	GCC trunk	Clang 16
3.9.1	✅	✅	✅	✅	✅
3.10.5	✅	✅	✅	✅	✅
3.11.1	❌	❌	❌	✅	✅
trunk	❌	❌	❌	✅	✅

Reproduction steps

https://godbolt.org/z/WbexYTqnf

Expected vs. actual results

std::views::filter should allow you to filter a json array.

Minimal code example

#include <nlohmann/json.hpp>
#include <iostream>
#include <ranges>

using nlohmann::json;

int main()
{
    auto noOpFilter = std::views::filter([](auto&&){ return true; });
    json j = {1, 2, 3};
    auto filtered = j | noOpFilter;
    std::cout << *filtered.begin() << std::endl;
    return 0;
}

Error messages

output.s: /opt/compiler-explorer/libs/nlohmann_json/trunk/single_include/nlohmann/json.hpp:13346: bool nlohmann::json_abi_v3_11_3::detail::iter_impl<BasicJsonType>::operator==(const IterImpl&) const [with IterImpl = nlohmann::json_abi_v3_11_3::detail::iter_impl<nlohmann::json_abi_v3_11_3::basic_json<> >; typename std::enable_if<(std::is_same<IterImpl, nlohmann::json_abi_v3_11_3::detail::iter_impl<BasicJsonType> >::value || std::is_same<IterImpl, nlohmann::json_abi_v3_11_3::detail::iter_impl<typename std::conditional<std::is_const<_Tp>::value, typename std::remove_const<_Tp>::type, const BasicJsonType>::type> >::value), std::nullptr_t>::type <anonymous> = nullptr; BasicJsonType = nlohmann::json_abi_v3_11_3::basic_json<>]: Assertion `m_object != nullptr' failed.
Program terminated with signal: SIGSEGV

Compiler and operating system

GCC 10.2

Library version

3.11.3

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.

[nlohmann]

In #4498 I added the code above as test case. I get the same assertion error in the job for GCC 10.5. Furthermore, the test does not compiler with Clang 11. The test succeed with GCC 13.0 and Clang 19. Interestingly, there are no errors on Windows and macOS.

To summarize (see https://github.com/nlohmann/json/actions/runs/11878152025):

Does not work:

• GCC 10.5.0 (assertion)
• Clang 11.1.0 (compilation error)
• Clang 12.0.1 (compilation error)
• Clang 13.0.1 (compilation error)
• Clang 14.0.6 (compilation error)
• Clang 15.0.7 (compilation error)

Works:

• GCC 11.5.0
• GCC 12.4.0
• GCC 13.3.0
• GCC 14.2.0
• Clang 16.0.6
• Clang 17.0.6
• Clang 18.1.8
• Clang 19.1.4

Test skipped:

• GCC 9.5.0

[nlohmann]

So the feature works as expected with reasonably modern versions of GCC and Clang. The assertion in GCC 10.5.0 feels to me like a shortcoming of the implementation of std::views::filter. Maybe there is some documentation about the implementation status of ranges in GCC that can confirm this.

For Clang, we see the features works as expected starting with Clang 16. The previous versions in the CI fail to compile the code. However, these tests rely on libstdc++ instead of libc++. Once I have some time, I will try to use libc++ with these compilers to check the behavior.

If there is nothing in the library to fix this, then we can use Hedley to detect non-compliant compilers to exclude the test case and add a section to the documentation/README about ranges support.

Any thoughts on this?