Can anyone explain this?

[Simen Kjærås]

On Tuesday, 5 June 2018 at 08:26:22 UTC, Nicholas Wilson wrote:
> writeln("Assert"); // this is not caught because the thrown 
> throwable is not an exception

Now that's plain false - If you replace the call to 
throwingFunc() with a string literal, you'll see it's properly 
caught. Also, the catch clause explicitly specifies AssertError, 
which as the name implies, is an Error, not an Exception.


This might just be a case of undefined behavior. According to 
https://dlang.org/spec/contracts.html:

"As a contract, an assert represents a guarantee that the code 
must uphold. Any failure of this expression represents a logic 
error in the code that must be fixed in the source code. A 
program for which the assert contract is false is, by definition, 
invalid, and therefore has undefined behaviour."

In short, what you're doing is UB, and the nasal demons are 
printing 'not gonna happen' to your console.

That also makes some sense with Andrea Fontana's comments:
> - Do not have side effects in either AssignExpression that 
> subsequent code depends on.
> - Do not attempt to resume normal execution after an Assert 
> Failure.


However, if we add catch (Throwable ex) to the list of catch 
statements, that *does* catch a regular object.Exception. Moving 
the catch (Exception) statement down the list does nothing. 
Adding another layer of try-catch also catches it (even with just 
catch (Exception ex)):

import std.stdio;
import core.exception;

unittest {
     scope(failure) {
         writeln("Not gonna happen");
     }

     try {
         try {
             static string throwingFunc() {
                 throw new Exception("An exception");
             }
             assert(0==1, throwingFunc());
         } catch(AssertError ex) {
             writeln("Assert");
         } catch(Exception ex) {
             writeln("Exception A");
         }
     } catch(Exception ex) {
         writeln("Exception B");
     }
}

So I'm stumped. It seems the exception handling is simply not 
setup correctly. This could be because of UB I guess, but I'd 
hope the AssertError would be thrown first in such a case.

--
   Simen