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For Apple clang-800.0.42.1, which was released with Xcode 8 in
September 2016, the '=delete' on the 'is' and 'as' methods causes
the following errors for value.h:

  inline declaration of 'as<bool>' follows non-inline definition
  inline declaration of 'is<bool>' follows non-inline definition

etcetera for the other specializations of 'is' and 'as'.  The same
problem also occurs for clang-3.8 but not clang-3.9 or later.