Change operator== overload that constrains to is_scalar_v to constrain to compatible types.

[fsandhei]

#4165 shows an issue with the overload resolution when attempting to compare a std::string with json from left to right. This can be fixed by changing the constraint to instead of focusing on scalar types, we constrain the comparison operator to "compatible types", in par with the catch-call constructor.

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[coveralls]

coverage: 100.0%. remained the same
when pulling 65b9b38 on fsandhei:fix/equality-operator-for-compatible-types
into 0457de2 on nlohmann:develop.

[nlohmann]

Can you please try to amalgamate with an older version of astyle, like astyle 3.2? There was a breaking change in astyle which makes the output differ unfortunately.

[fsandhei]

Can you please try to amalgamate with an older version of astyle, like astyle 3.2? There was a breaking change in astyle which makes the output differ unfortunately.

Sure, I'll do it some time later today after work.

[fsandhei]

First, sorry, @nlohmann, for the trouble I'm causing here.

So, I tried amalgamating again. There seems to be some issues or inconsistencies with the astyle version 3.2.1 from the AUR versus 3.2.1-build from apt:

• --squeeze-lines is not defined in the one from Arch, so I had to
  comment that one out in order to make the amalgamate step run.
• As far as I see, squeeze-line was introduced in 3.3.x.
• A bunch of files are affected by the amalgamate, which does not
  seem right.

Would you want me to perhaps unstage all the other unrelated files that got affected by this?

[github-actions]

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[nlohmann]

This is what I am using locally (patched Makefile):

# call the Artistic Style pretty printer on all source files
pretty:
	/Users/niels/Downloads/astyle/build/astyle \
	    --style=allman \
	    --indent=spaces=4 \
	    --indent-modifiers \
	    --indent-switches \
	    --indent-preproc-block \
	    --indent-preproc-define \
	    --indent-col1-comments \
	    --pad-oper \
	    --pad-header \
	    --align-pointer=type \
	    --align-reference=type \
	    --add-braces \
	    --convert-tabs \
	    --close-templates \
	    --lineend=linux \
	    --preserve-date \
	    --suffix=none \
	    --formatted \
	   $(SRCS) $(TESTS_SRCS) $(AMALGAMATED_FILE) $(AMALGAMATED_FWD_FILE) docs/examples/*.cpp

with Astyle 3.1.

[fsandhei]

Thank you for so quick response. The amalgamation seems happy now.

[nlohmann]

Thank you for so quick response. The amalgamation seems happy now.

Thanks for your effort!

[fsandhei]

I see that the GCC 10 CI run failed. It seems to be not able to parse the concept, which is odd as it should be supported in GCC 10.

For the sake of getting this in and have the least amount of hassle I don't use concept.

However I was pondering on the idea of having a concept which could replace these type traits definitions for recognizing "compatible types". I tinkered with this simple concept below. It worked perfectly fine. I don't know if it is missing any details, though. Would appreciate hearing your opinion on this, as I'm far from an expert :)

#ifdef JSON_HAS_CPP_20
   #include <concepts>
    template <typename T, typename BasicJsonType, typename U = uncvref_t<T>>
    concept CompatibleType = requires(const U& type, BasicJsonType& j) {
        { to_json(j, type) } -> std::convertible_to<void>;
    };
#endif

The use of this would be quite simple, as we'd then can constrict constructors / operators with this concept:

template <CompatibleType<basic_json_t> T>
operator==(T rhs) const noexcept
{
   ...
}

EDIT: I realize I goofed myself. It is supported but JSON_HAS_CPP_20 is not defined because g++10 requires -std=c++2a. I'm able to get past this by also checking for __cpp_concepts to be defined at where we define the concept.

[fsandhei]

I'm not really sure what is Codacy is actually complaining about. The struct member is being used in the type trait. It might be a bug in Codacy. Do you have any suggestions @nlohmann?

[nlohmann]

I'm not really sure what is Codacy is actually complaining about. The struct member is being used in the type trait. It might be a bug in Codacy. Do you have any suggestions @nlohmann?

Indeed looks like a false positive. I set it to "ignore".

[github-actions]

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