Enable CMake policy CMP0077

[alexreinking]

Projects that import json via FetchContent or add_subdirectory pointed at a git submodule may want to set JSON_BuildTests to "NO". However, this doesn't work without creating an identical option() in the importing project. Enabling CMP0077 in CMake 3.13+ changes the behavior of option() to allow importing projects to set default values for the variables without touching the cache.

See the documentation for CMP0077 here: https://cmake.org/cmake/help/latest/policy/CMP0077.html

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

[coveralls]

Coverage remained the same at 100.0% when pulling ec43371 on alexreinking:patch-1 into 635b9a0 on nlohmann:develop.

[t-b]

@alexreinking Can you clarify why that is needed?

I'm doing here

SET(JSON_BuildTests OFF CACHE INTERNAL "")
SET(JSON_Install OFF CACHE INTERNAL "")
ADD_SUBDIRECTORY(json)

and the JSON tests are not built and I also don't define these options. My cmake version is 3.15.2 on windows.

[alexreinking]

The key is not touching the cache. The snippet you posted will no longer set JSON_BuildTests to OFF if I explicitly set it to "ON" in the cache. Additionally, option() will not create a cache entry at all with CMP0077 enabled and the normal variable set.

In CMake 3.13 and above the option() command prefers to do nothing when a normal variable of the given name already exists. It does not create or update a cache entry or remove the normal variable. The new behavior is consistent between the first and later runs in a build tree. This policy provides compatibility with projects that have not been updated to expect the new behavior.

Without the policy, the following does not work, but should, and is the default behavior of CMake going forward:

set(JSON_BuildTests OFF)
set(JSON_Install OFF)
add_subdirectory(json)

[nlohmann]

Looks good to me.

[nlohmann]

Thanks!

[nlohmann]

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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

• Enable CMake policy CMP0077. Enable CMake policy CMP0077 #2222

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[t-b]

@alexreinking Thanks for the clarification. Makes sense!