Compilation failure of tests with C++20 standard (caused by change of u8 literals)

[stac47]

Compiling tests with gcc and the -std flag set to gnu++20 or c++20, leads to compilation issues.

What is the issue you have?

More precisely, the u8"" literals have changed with the C++20 standard via P0482 in a non backward-compatible way char8_t is not the same as unsigned char.

When we compile the test suite of the project we get the following kind of errors with gcc:

In file included from json-3.9.1/test/thirdparty/doctest/doctest_compatibility.h:6,
                 from json-3.9.1/test/src/unit-class_parser.cpp:30:
json-3.9.1/test/thirdparty/doctest/doctest.h: In instantiation of 'doctest::detail::Result doctest::detail::Expression_lhs<L>::operator==(const R&) [with R = char8_t [5]; L = const std::__cxx11::basic_string<char>&]':
json-3.9.1/test/src/unit-class_parser.cpp:513:17:   required from here
json-3.9.1/test/thirdparty/doctest/doctest.h:1119:32: error: no match for 'operator==' (operand types are 'const std::__cxx11::basic_string<char>' and 'const char8_t [5]')
 1119 | #define DOCTEST_CMP_EQ(l, r) l == r
      |                                ^

This comes from the following type of line which is not correct now:

 CHECK(parser_helper("\"\\ud80c\\udc60\"").get<json::string_t>() == u8"\U00013060");

As there are many ways to cope with this in the code (for instance described here, I let the maintainers decide which one fits the best.

Please describe the steps to reproduce the issue.

Build it the standard way

mkdir -p _build
cd _build
cmake ..
make V=1 -j8

Relevant environment variables:

• CXXFLAGS=' -std=gnu++20 -fno-working-directory -ggdb3'
• LDFLAGS=' -fno-working-directory -ggdb3'

Can you provide a small but working code example?

We can warkaround the problem by disabling the char8_t support with the -fno-char8_t compiler option.

What is the expected behavior?

Compilation with no error

And what is the actual behavior instead?

See above.

Which compiler and operating system are you using?

• Compiler: gcc 11 (developement version)
• Operating system: GNU/Linux ubuntu 20.04

Which version of the library did you use?

• [x ] latest release version 3.9.1
• other release - please state the version: ___
• the develop branch

If you experience a compilation error: can you compile and run the unit tests?

• yes
• [x ] no - please copy/paste the error message below

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[nlohmann]

@stac47 Thanks for reporting. I did not test the library with C++20 so far. The issue should be fixed with #2550. Can you please check the latest develop branch?