Unreachable code: error or warning? [closed]

Question

This is a language design question:

Do you think unreachable code (in programming languages in general) should raise a warning (i.e. "report problem and compile anyway") or an error ("refuse to compile")?

Personally I strongly feel it should be an error: if the programmer writes a piece of code, it should always be with the intention of actually running it in some scenario. But the C# compiler for example seems to disagree with this and merely reports a warning.

Note: I realize good dead code detection is a very difficult problem, but that is not the focus of this question.

Here are some examples of pieces of code where some statements are clearly unreachable:

return;
foo();

--

throw new Exception();
foo();

--

if (...) {
  return;
} else {
  throw new Exception();
}
foo();

Answer 1

An error means that the compiler is physically unable to deal with your code.

A warning means that the compiler is capable of dealing with your code, but it believe that what you have written is wrong in some way.

It seems pretty clear cut to me - it should be a warning.

Besides, what about the case where I've decided to shorten a method for debugging purposes:

public bool ShowMenu()
{
    return true;
    /* The standard implementation goes here */
}

I know its wrong, but for the compiler to ask me to also comment out that code would just be a pain.

Answer 2

It is very common to create dead code intentionally when debugging - a compiler that prevents this is unlikely to be popular with its users.