Code annotation for Exceptions

[linkdotnet]

Problem

In the current setup, there is no way of telling that a method will throw an exception so that the code after will not be executed. A small example:

bool Throw()
{
    try
    {
        ActionWhichMightThrow();
        return true;
    }
    catch (Exception)
    {
        AlwaysThrow();

        // Never reached, but we have to add this
        throw;
    }
}

[DoesNotReturn]
void AlwaysThrow() => throw new Exception();

Even though AlwaysThrow will never return and will throw an exception the method would not be compilable without the additional (and useless) throw at the end (or a return statement).

Real-world example

I encountered this problem most often in combination with ExceptionDispatchInfo:

try
{
    return waiter.WaitTask.GetAwaiter().GetResult();
}
catch (AggregateException e) when (e.InnerExceptions.Count == 1)
{
    ExceptionDispatchInfo.Capture(e.InnerExceptions[0]).Throw();

    // Unreachable code
    throw;
}

New behavior

We already have DoesNotReturn and DoesNotReturnIf which could be a good beginning. If a method is flagged by this annotation the compiler should understand that the code afterward will never be reached. In its current state DoesNotReturnIf only understands a boolean input, maybe a new DoesNotReturnIfNull would also be helpful.

PS: I did neither add this as a proposal nor as a bug as I wasn't sure what category fits best.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[KalleOlaviNiemitalo]

Making DoesNotReturnAttribute affect definite assignment and reachability was rejected in dotnet/csharplang#2555 (comment) and dotnet/csharplang#3826 (comment). Alternative C# language changes have been discussed in dotnet/csharplang#538 and dotnet/csharplang#739.

[linkdotnet]

Thanks for the swift response @KalleOlaviNiemitalo
seems like a duplicate of dotnet/csharplang#739. Thanks. Will close the issue

[KalleOlaviNiemitalo]

maybe a new DoesNotReturnIfNull would also be helpful

NotNullAttribute on an input parameter already means that, if the function returns, then the compiler can assume that the argument was not null.

[linkdotnet]

True but not in the context of an exception (or whether we can continue the control flow after the method in question).