C# Generic Type Inference With Multiple Types

My question is, why can't TInput be inferred in this situation?

It can - it's TResult which can't be inferred, and there's no way of specifying "partial" inference.

What you can sometimes do is separate the type parameters into ones for a generic type and ones for a generic method, so you'd end up with:

// Explicitly state TResult, and infer TInput
Serializer<MySuperType>.Serialize(x);

Why not just write it like this:

public string SerialiseAs<TResult>(TResult input)
{
    DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(TResult));
    MemoryStream stream = new MemoryStream();
    ser.WriteObject(stream, input);
    stream.Position = 0;
    StreamReader reader = new StreamReader(stream);
    return reader.ReadToEnd();
}

Since TInput derives from TResult, you really don't need to specify at all.