'Unreachable Statement' Compile Error in Apex When Using API Version 42.0 or Higher

Understanding the Behavior Change

Old behavior (pre-API Version 42.0):
The old Apex compiler allowed dead code — that is, code that could never be reached at runtime — to compile without error. For example, a method that throws an exception followed by a call to another method would compile even though the second method call is unreachable. The old compiler also allowed mutation of final variables.

void m() {
throw new MyException();
doSomething();
}

New behavior (API Version 42.0 and later):
The new compiler rejects unreachable statements and mutations of final variables. Code placed after an unconditional throw statement is flagged as unreachable and the class will not compile.

void m() {
if (true) {
throw new MyException();
}
doSomething();
}

How to Fix the Compile Error

To resolve the "Unreachable statement" compile error, use one of the following approaches:
Option 1 — Remove the dead code:
Review the method and remove any statements that can never be reached. This is the recommended approach as it improves code clarity and maintainability.
Option 2 — Use a conditional wrapper (compatibility workaround):
If removing the dead code is not immediately possible, wrap the throw statement in a conditional block such as if (true) { throw new MyException(); }, followed by the originally unreachable statement. This satisfies the new compiler's reachability analysis while preserving the intended logic. Note: This is a workaround for backward compatibility — the best practice is to refactor the code to remove dead code paths entirely.
For final variable mutation errors:
Remove any operations that mutate a final variable after its declaration. A final variable should be assigned only once at the point of declaration. If the variable value needs to change, remove the final keyword.