Does not compute

One of the most basic ways to think about a computer program is that it is a device which takes in integers as inputs and spits out integers as outputs. The C# compiler, for example, takes in source code strings, and those source code strings are essentially nothing more than enormous binary numbers. The output of the compiler is either diagnostic text, or strings of IL and metadata, which are also just enormous binary numbers.  Because the compiler is not perfect, in some rare cases it terminates abnormally with an internal error message. But those fatal error messages are also just big binary numbers. So let’s take this as our basic model of a computer program: a computer program is a device that either (1) runs forever without producing output, or (2) computes a function that maps one integer to another.
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Never say never, part two

This is part two of a two-part series about determining whether the endpoint of a method is never reachable. Part one is here. A follow-up article is here.


Whether we have a “never” return type or not, we need to be able to determine when the end point of a method is unreachable for error reporting in methods that have non-void return type. The compiler is pretty clever about working that out; it can handle situations like

int M()
{
  try
  {
    while(true) N();
  }
  catch(Exception ex)
  {
    throw new WrappingException(ex);
  }
}

The compiler knows that N either throws or it doesn’t, and that if it doesn’t, then the try block never exits, and if it does, then either the construction of the exception throws, or the construction succeeds and the catch throws the new exception. No matter what, the end point of M is never reached.

However, the compiler is not infinitely clever. It is easy to fool it:
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