Disallowing implicit conversion from boolean type to some integers type (#4358)

[arifghaz]

Abstract

Referring to issue #4358 , this pull request make some changes that causes implicit conversions from various integer types (listed here - https://godbolt.org/z/7Wrh6EanW) to boolean to fail. Now, instead of inconsistence behavior for different integer types, all implicit conversion from boolean to different integer types will throw a type error.

Changes Proposed

Remove explicit overload for boolean_t from the from_json function (line 343 from_json.hpp). This will allow all implicit conversions from boolean to integers to fail.

Possible Issues

This removal will causes two unit tests in unit-conversions.cpp to fail, but both unit tests involve implicit conversion from boolean to uint8_t (line 556-560). Based on issue #4358 , the failure of these two unit tests are expected. Removal of these tests allow the changes to pass all the remaining test cases.

Validation

Unit test will be added if the propose change is accepted.

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Pull request checklist

Read the Contribution Guidelines for detailed information.

• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Specifically, I am aware of compilation problems with Microsoft Visual Studio (there even is an issue label for this kind of bug). I understand that even in 2016, complete C++11 support isn't there yet. But please also understand that I do not want to drop features or uglify the code just to make Microsoft's sub-standard compiler happy. The past has shown that there are ways to express the functionality such that the code compiles with the most recent MSVC - unfortunately, this is not the main objective of the project.
• Please refrain from proposing changes that would break JSON conformance. If you propose a conformant extension of JSON to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[nlohmann]

This change breaks the public API - existing code (such as the tests) must not be affected by the change. For this issue, a preprocessor macro would be needed. Not sure if this is the best approach. This should be discussed in #4358.

[coveralls]

coverage: 99.649%. remained the same
when pulling 567c2d2 on amirghaz:pr2
into 1b9a9d1 on nlohmann:develop.