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Fix and add test's for SFINAE problem #1741

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merged 1 commit into from
Sep 16, 2019
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@tete17 tete17 commented Sep 7, 2019

This Pull Requests fixes #1727
And adds a test that would have not worked before to avoid regressions 😄

This was simply due to a pretty typical SFINAE bug. Since the function signature was

bool contains(KeyT && key) const

When a rvalue is passed (say int for simplicity reasons) you want this to be transformed into

bool contains(int && key) const

So far so good, trouble begins when given a lvalue, since the end result should be

bool contains(int & key) const

This can only be achieved by the special rule of type deduction in c++ where:

  • & + && -> &

In conclussion:
KeyT && key -> int & && key -> int & key

More specific to the issue KeyT was being deducted to json::json_pointer & which is clearly not the same as json::json_pointer so our lovely friend the std::enable_if wasn't doing his job properly.

Decaying the type should fix this 😄

God... metaprogramming is hard...


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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 file single_include/nlohmann/json.hpp. The whole process is described here.

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  • 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.
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coveralls commented Sep 7, 2019

Coverage Status

Coverage remained the same at 100.0% when pulling e26a290 on tete17:Fix-SFINAE into 06ccd43 on nlohmann:develop.

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Could you please add the code from #1727 (comment) as regression test? Otherwise, the code looks find and I am looking forward to merging! Thanks!

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tete17 commented Sep 10, 2019

The Ci broke for Xcode 8.3 because of hombrew 😢

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The Ci broke for Xcode 8.3 because of hombrew 😢

I restarted the job and it succeeded now.

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Looks good to me.

@nlohmann nlohmann self-assigned this Sep 16, 2019
@nlohmann nlohmann added this to the Release 3.7.1 milestone Sep 16, 2019
@nlohmann nlohmann merged commit 771d5da into nlohmann:develop Sep 16, 2019
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Thanks a lot!

@tete17 tete17 deleted the Fix-SFINAE branch September 16, 2019 20:41
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🔖 Release item

This issue/PR will be part of the next release of the library. This template helps preparing the release notes.

Type

  • ✨ New Feature
  • 🐛 Bug Fix
  • ⚡️ Improvement
  • 🔨 Further Change
  • 🔥 Deprecated function

Description

  • Fixed a SFINAE problem in the contains function for JSON Pointers that led to different results for const/nonconst arguments.

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json::contains usage to find a path
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