Mac OS 升级10.15之后，objc-750编译不过

报找不到_objc_opt_class

Undefined symbols for architecture x86_64:
  "_objc_opt_class", referenced from:
      -[NSObject superclass] in NSObject.o
      -[NSObject isMemberOfClass:] in NSObject.o
      -[NSObject isKindOfClass:] in NSObject.o
      +[NSObject isAncestorOfObject:] in NSObject.o
      -[NSObject respondsToSelector:] in NSObject.o
      -[NSObject conformsToProtocol:] in NSObject.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

简单快速的方法：General -> Deployment Target 改为10.14.

Pass!