Last/First + xxxOrDefault variants should check against IReadOnlyList, not mutable IList

[marcus-sonestedt]

See the line below:

runtime/src/libraries/System.Linq/src/System/Linq/Last.cs

Line 78 in 74cf618

if (source is IList<TSource> list)

IReadOnlyList has the indexer, which is the only thing this code needs, IIUC.

Thanks for an awesome framework! :)

[MihuBot]

I'm a bot. Here are possible related and/or duplicate issues (I may be wrong):

• Enumerable methods with optimizations for IList<> should also optimize for IReadOnlyList<> #20812
• System.Linq.Enumerable has special treatment of IList<T>, which should also apply to IReadOnlyList<T> #57822
• Respect IReadOnlyList<T> in the BCL #24793

[marcus-sonestedt]

Yup, duplicate of all the above.

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