Cannot add #if compiler(>=5.0) in `AppDelegate.m` file: - ios

Question

Trying to add a block in the AppDelegate.m file
#if compiler(>=5.0)
if (#available(iOS 13.0, *)) {
// do something...
}
#endif
The error showed is:
Function-like macro 'compiler' is not defined

Answer 1

The following is a Swift compile-time directive:
#if compiler(>=5.0)
...
#endif
It makes no sense to use this in an Objective-C file, AppDelegate.m. In an Objective-C source, it will result in the “Function-like macro 'compiler' is not defined” error.
So, if you want a runtime check for iOS 13, just remove the #if, e.g.:
if (#available(iOS 13.0, *)) {
...
}
See Marking API Availability in Objective-C.
A couple of observations (given that you tagged this with swift):
In Swift, the compile-time pattern is:
#if swift(>=5.0)
...
#endif
Or
#if compiler(>=5.0)
...
#endif
This pattern is generally only used when writing third-party libraries where you need to offer backward support to developers who are compiling your library with earlier versions of Swift.
See The Swift Programming Language: Compiler Control Statements
The following is an Objective-C runtime check for OS version:
if (#available(iOS 13.0, *)) {
// do something...
}
The equivalent Swift pattern is:
if #available(iOS 13.0, *) {
...
}
These are useful when writing code that is targeting multiple iOS versions, but you want to conditionally run code using iOS 13+ API.
To make it confusing, Swift does have an #available construct, but it is for marking a class, extension, or method as being available only for devices running a particular OS version, but is not used in conjunction with an if test. E.g. let’s say you are defining some function that should only be available to iOS 13 and later:
#available(iOS 13.0, *)
func someMethod() {
...
}
This is compile-time construct is to tell the compiler to not complain about iOS 13 API within this function if your app happens to support earlier iOS versions. You are effectively telling the compiler “hey, I know this has code inside it that only works with iOS 13 and later, but don’t worry, because I’ll only call it when the device in question is running iOS 13 or later.”

Answer 2

There are many things wrong that you did in your code example:
#if is a preprocessor macro. You can only use it outside the scope of anything, just like the import statement. If you are using it the way it should be used, then you can't use that if statement there.
#available cannot be used inside of an if statement. It can only be used to represent if a particular class/struct/func is available for said OS version and up, but not fragments of code. For that, use this instead:
if #available(iOS 13.0, *) {
// Do something.
}

Answer 3

If you want to check the Swift compiler version specifically, you can use #if swift. To check for iOS version during runtime, you need to use #available, the # version if used to mark types/functions only available on said versions.
#if swift(>=5.0)
if #available(iOS 13,*) {
//do something
}
#endif