func assertionFailure(_: () -> String, file: StaticString, line: UInt)

Indicate that an internal sanity check failed.

Use this function to stop the program, without impacting the performance of shipping code, when control flow is not expected to reach the call (e.g. in the default case of a switch where you have knowledge that one of the other cases must be satisfied). To protect code from invalid usage in Release builds; see preconditionFailure.

  • In playgrounds and -Onone builds (the default for Xcode's Debug configuration) stop program execution in a debuggable state after printing message.

  • In -O builds, has no effect.

  • In -Ounchecked builds, the optimizer may assume that this function will never be called. Failure to satisfy that assumption is a serious programming error.


func assertionFailure(@autoclosure message: () -> String = default, file: StaticString = default, line: UInt = default)