I think this was included for compatibility with IronPython. How would you handle out parameters that still have automatic conversions like int and str?

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Merging #1308 (ff89811) into master (57f4a80) will decrease coverage by 13.84%.
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@filmor this is the confusing part. This change does not affect GetInt(out int val) functions. It affects GetInt([Out] int val) functions, where [Out] parameter does nothing. The only real use case I see is that in Read([Out] byte[] buf) [Out] indicates, that the data will be put into the buf. The data can not be put into string or int without ref int/out int, and these are handled separately.

This annotation is normally meant for P/Invoke marshaling scenarios, where you have to add [Out] to byte[] arr into PInvoke signature for data to be properly marshaled back into arr upon exiting C function. I am not even sure why MemoryStream.Read has this annotation except for informational purposes. I believe passing Python lists in place of buf is why it was added, but considering lack of [Out] annotations in most places, I don't think it is a good thing to rely on.

For an instance of API difference between implementations in regards to [Out] attribute:

In .NET Core MemoryStream.Read has none: https://github.com/dotnet/runtime/blob/5432a66018c2499b88a54276f468ea903ed2b8de/src/libraries/System.Private.CoreLib/src/System/IO/MemoryStream.cs#L333

In Mono it is present for COM-compatibility: https://github.com/mono/mono/blob/c5b88ec4f323f2bdb7c7d0a595ece28dae66579c/mcs/class/referencesource/mscorlib/system/io/memorystream.cs#L339

Without this change the same Python code would not work with both Mono and .NET Core.

Thanks for the explanation. I ran into this problem while testing .NET Core and couldn't really make sense of it, wasn't aware of [Out].