Please add APIs to determine the Unicode category of a UTF32 code point

[gafter]

Please add an API for determining the Unicode category of a code point resulting from a surrogate pair. Presumably I would compute that by starting with

        public static int System.Char.ConvertToUtf32(char highSurrogate, char lowSurrogate) {

But there is no way to get the unicode category of the resulting code point.

The closest thing we have right now is this:

        public static UnicodeCategory System.Char.GetUnicodeCategory(String s, int index) {

But that requires that we store the surrogates into a newly created string in order to get the Unicode category. That requires an unacceptably high allocation overhead.

The original problem I am trying to solve is dotnet/roslyn#9731, dotnet/roslyn#13474, and dotnet/roslyn#13560.

[stephentoub]

dotnet/coreclr#15911 added:

public static UnicodeCategory GetUnicodeCategory(int codePoint)

to CharUnicodeInfo, available in .NET Core 2.1. This was previously an internal method named InternalGetUnicodeCategory.

[gafter]

Perfect! Thanks.