Feature nullness :: Subsume nullness for contravariant type parameters (such as the one in IEqualityComparer<in T>) #17398
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This feature is driven by an occurence of BCL type design which I encountered during testing.
IEqualityComparer<in T>StringComparer.Ordinal :
IEqualityComparer<string | null>But constructors of all dictionary-like types needing an
IEqualityComparer<string>Which would not much.
At the same time, natural conversion from Type<T | null> to Type cannot be done out of the box, since we cannot control how the
Tis used internally.This nullness-equality relaxation can only be done for contravariant generic parameters (
<in T>), which is used for interfaces taking the generic typar in its input.