RSRP-288523: Suggesting OfType on generic collections

Given this code:

IEnumerable<Stream> streams = new Stream[] {new MemoryStream(), new BufferedStream(new MemoryStream()), };
var memoryStreams = streams.Where(x => x is MemoryStream).Select(x => (MemoryStream) x);

ReSharper 6.1.42.60 suggests replacing the second line with OfType<T>:

var memoryStreams = streams.OfType<MemoryStream>();

Now replace the definition of 'streams' with:

IEnumerable<string> streams = new[] {""};

The code with OfType compiles successfully, even though it's clearly incorrect. The original code wouldn't have compiled, the cast from string to MemoryStream fails at compile-time.

In more general terms, under these conditions:

IEnumerable<Supertype> x
x.Where(x => x is Subtype).Select(x => (Subtype) x)

ReSharper shouldn't recommend replacing with x.OfType<Subtype>()

Even more generally, ReSharper should never recommend applying OfType<T> to an IEnumerable<U> when U != object.
It should offer to convert code away from OfType<T> when it sees it being applied to an IEnumerable<U>, either to the original code posted here or:

streams.Select(x => x as MemoryStream).Where(x => x != null);

which is also type-safe and a bit less verbose.

Related: RSRP-288411 , RSRP-288336