The type parameter 'TQueryType' cannot be used with the 'as' operator because it does not have a class type constraint nor a 'class' constraint

[marwie]

Input code

Please see #2389

Relevant section in official CSReference
https://github.com/Unity-Technologies/UnityCsReference/blob/61f92bd79ae862c4465d35270f9d1d57befd1761/Modules/Subsystems/SubsystemManager.cs#L35

Erroneous output

<UnityEngine>\UnityEngine\SubsystemManager.cs:3536 The type parameter 'TQueryType' cannot be used with the 'as' operator because it does not have a class type constraint nor a 'class' constraint

private static void AddSubsystemSubset<TBaseTypeInList, TQueryType>(List<TBaseTypeInList> copyFrom, List<TQueryType> copyTo) where TBaseTypeInList : ISubsystem where TQueryType : ISubsystem
{
	TQueryType item = default(TQueryType);
	foreach (TBaseTypeInList item2 in copyFrom)
	{
		int num;
		if (item2 is TQueryType)
		{
			object obj = item2;
			item = (TQueryType)(obj as TQueryType);
			num = 1;
		}
		else
		{
			num = 0;
		}
		if (num != 0)
		{
			copyTo.Add(item);
		}
	}
}

Same issue for AddDescriptorSubset https://github.com/Unity-Technologies/UnityCsReference/blob/61f92bd79ae862c4465d35270f9d1d57befd1761/Modules/Subsystems/SubsystemDescriptorStore.cs#L29

Details

Please see #2389

[dgrunwald]

The IL code is weird here.
It uses this sequence for the actual cast:

			IL_0021: ldloc.1
			IL_0022: box !!TBaseTypeInList
			IL_0027: isinst !!TQueryType
			IL_002c: unbox.any !!TQueryType
			IL_0031: stloc.2

The isinst in there seems to be completely redundant given that item2 is TQueryType was already checked by the if-condition above.
But ILSpy is translating this snippet locally, so it doesn't know that the type is guaranteed to match.
And if the type does not match, the weird isinst changes the semantics: instead of the InvalidCastException that you'd typically get, you would get a NullReferenceException because the isinst replaces non-matching types with a null reference.

So I don't think we can easily fix this :(

[dgrunwald]

I can reproduce the weird IL sequence with:

        public void Generic<T1, T2>(T1 x) where T2 : IDisposable
        {
            if (x is T2 y)
            {
                y.Dispose();
            }
        }

I think we would first need to implement support for pattern matching, as it's only in the context of the whole pattern that we could detect that the second isinst is redundant.

[dgrunwald]

Actually we had this topic before for non-generic types: #1903
There we came up with a workaround:
item = (TQueryType)(obj as TQueryType);
can be expressed as
item = (TQueryType)(obj is TQueryType ? obj : null);

Not sure why the workaround doesn't trigger for generic types, but that might be easy to fix.

[marwie]

Oh nice, thanks for the info and fix. I'll try the next few days again!