C#8 compiler reports CS8620 for IEnumerable<T?> after filtering for not null

[Bonuspunkt]

Version Used:
dotnet-sdk-3.0.100

Steps to Reproduce:
enable nullable

public static void Run()
{
    var enumerable = Enumerable.Range(1, 10).Select(i => i % 2 == 0 ? null : i.ToString());
    var notNulls = enumerable.Where(item => item != null);
    Check(notNulls);
}
private static void Check(IEnumerable<string> notNull) { }

Expected Behavior:
no warning

Actual Behavior:
Warning CS8620 Argument of type 'IEnumerable<string?>' cannot be used for parameter 'notNull' of type 'IEnumerable' in 'void Check(IEnumerable notNull)' due to differences in the nullability of reference types.

my current workaround is

    var notNulls = enumerable.Where(item => item != null).Select(item => item!);

[huoyaoyuan]

Where(x => x != null) is quite hard to deal for nullable analysis.
Potential solutions:

• A concrete method IEnumerable<T> WhereNotNull<T>(IEnumerable<T?>), using yield return itself.
• Use OfType<T>. The semantic of OfType equals to null-check when applied to the same reference type.

[RikkiGibson]

Perhaps a duplicate of dotnet/csharplang#8383

[jcouv]

Yes, this is a duplicate of dotnet/csharplang#8383
I'll go ahead and close as such. Thanks