New in version 3.7.
Set to the Swift language version number. If not set, the oldest legacy version known to be available in the host Xcode version is assumed:
4.0 for Xcode 10.2 and above.3.0 for Xcode 8.3 and above.2.3 for Xcode 8.2 and below.
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Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.19/variable/CMAKE_Swift_LANGUAGE_VERSION.html