explicit conversion to string not considered for std::map keys in GCC8

[ottojo]

• What is the issue you have?
  Compilation fails with GCC8 when trying to serialize a std::map whose Key-type provides explicit conversion to std::string

• Please describe the steps to reproduce the issue. Can you provide a small but working code example?

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

class ID {
public:
    explicit ID(int i) : id(i) {};

    explicit operator std::string() const {
        return std::to_string(id);
    }

    bool operator<(const ID &rhs) const {
        return id < rhs.id;
    }

private:
    int id;
};

int main() {
    std::map<ID, std::string> map = {
            {ID(7),  "test1"},
            {ID(13), "test2"}
    };

    nlohmann::json mapJson = map;
    std::cout << mapJson.dump(4) << std::endl;
 }

• What is the expected behavior?
  Serialization to

{
    "13": "test2",
    "7": "test1"
}

• And what is the actual behavior instead?
  Does not compile when using gcc version 8

g++-8 (Ubuntu 8.4.0-1ubuntu1~18.04) 8.4.0

Compiler error:

no known conversion for argument 2 from ‘std::pair<const ID, std::__cxx11::basic_string<char> >::first_type’ {aka ‘const ID’} to ‘const key_type&’ {aka ‘const std::__cxx11::basic_string<char>&’}

• Which compiler and operating system are you using? Is it a supported compiler?

  • I found this issue only when compiling with GCC 8.4.0
  • Other compilers tested:
    • g++-9 (Ubuntu 9.3.0-11ubuntu0~18.04.1) 9.3.0
    • g++-10 (Ubuntu 10-20200416-0ubuntu1~18.04) 10.0.1 20200416 (experimental) [master revision 3c3f12e2a76:dcee354ce56:44b326839d864fc10c459916abcc97f35a9ac3de]
    • clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
    • clang version 11.0.0-++20200506091409+ca09dab303f-1~exp1~20200506072011.1666

• Did you use a released version of the library or the version from the develop branch?

  • Version 3.7.3 from develop branch

• unit tests compile and run successfully

[nlohmann]

I can run the example code without problems.

Output:

{
    "13": "test2",
    "7": "test1"
}

• Branch: current develop branch
• Compiler: Xcode Version 11.4.1 (11E503a)
• OS: macOS Catalina 10.15.4 (19E287)

I'll try to check GCC 8.

[nlohmann]

I tried some more compilers (all on macOS):

• [FAIL] g++-6 (Homebrew GCC 6.5.0_5) 6.5.0
• [FAIL] g++-7 (Homebrew GCC 7.5.0_2) 7.5.0
• [FAIL] g++-8 (Homebrew GCC 8.4.0_1) 8.4.0
• [SUCCESS] g++-HEAD (Homebrew GCC HEAD-705510a) 10.0.1 20200405 (experimental)

[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.