Detect unsupported SDK calls
Apple's advice for supporting older versions of the SDK is to set the deployment target, but link to the latest SDK. This is very dangerous in practice because if your program makes a call to a function that is not supported in your deployment target, you will get no warning, but it will crash at runtime. Neither the compiler nor the static analyzer will help protect you from this.
Apple's advice here is not useful, and developers in practice must link against the old SDK in order to avoid crashing.
Apple should provide a tool to detect SDK usage that is not compatible with the deployment target (including filtering out correct usage via weak linking, as Apple recommends). Until Apple can provide such a tool, Apple should stop removing old SDKs from Xcode.
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