F# emits catch/rethrow for exception type tests

[mattmccutchen-microsoft]

When the F# compiler translates a try-with block, by default it seems to generate IL that catches Object and then rethrows the exception if it doesn't match any of the patterns. (Here's the relevant source code.) This has the unpleasant effect that when the Visual Studio debugger reports a user-unhandled exception, it reports it at an unrelated enclosing catch block instead of where it was originally thrown. This was previously reported on Stack Overflow.

Repro steps

Run the following code under the debugger:

exception MyException of string

[<EntryPoint>]
let main argv =
  try
    failwith "oops"
  with
    | MyException _ -> ()
  0 // return an integer exit code

Expected behavior

The exception popup appears on the "failwith" call.

Actual behavior

The exception popup appears on the "with" block, even though it doesn't catch this type of exception.

Known workarounds

In the project properties, "Build" tab, "Other flags", add --generate-filter-blocks.

Related information

• Operating system: Windows 10 Enterprise, Version 1703, OS Build 15063.413
• Nightly Visual F# Tools 15.4.1.17063001 installed over the following Visual Studio:

Microsoft Visual Studio Enterprise 2017 
Version 15.2 (26430.14) Release
VisualStudio.15.Release/15.2.0+26430.14
Microsoft .NET Framework
Version 4.7.02046

Installed Version: Enterprise

Visual F# 4.1   00369-90050-01272-AA637
Microsoft Visual F# 4.1

[mattmccutchen-microsoft]

After reading this thread, maybe the answer is just "use --generate-filter-blocks (and suppress the deprecation warning) if you want". But is there a way this could be made discoverable to users the first time they see the exception popup at a catch block? An alternative would be to at least generate typed catch blocks for parity with C#.

[dsyme]

@mattmccutchen-microsoft What are your "Just My Code" settings (Under Options --> Debug)?

[0x53A]

I think this bugreport, and the other thread are distinct:

This one is about type-matching an exception, the other one is about exception filters. (MyException ex when ex ...).

Just for clarity, this is the equivalent C# code for the above F#:

	[EntryPoint]
	public static int main(string[] argv)
	{
		try
		{
			Unit unit = Operators.FailWith<Unit>("oops");
		}
		catch (object obj)
		{
			Exception ex = (Exception)obj;
			Program.MyException ex2 = ex as Program.MyException;
			if (ex2 == null)
			{
				throw;
			}
		}
		return 0;
	}

[dsyme]

@KathleenDollard I've labelled this as part of the "Simple F#" agenda since the experience of debugging uncaught exceptions is really important for the beginner user.