[Version 11.0] Feature support for pattern match span char on constant string

standard/patterns.md

@@ -148,6 +148,7 @@ For a constant pattern `P`, its *converted value* is
 Given a pattern input value *e* and a constant pattern `P` with converted value *v*,
 
 - if *e* has integral type or enum type, or a nullable form of one of those, and *v* has integral type, the pattern `P` *matches* the value *e* if result of the expression `e == v` is `true`; otherwise
+- if *e* is of type `System.Span<char>` or `System.ReadOnlySpan<char>`, and *v* is a constant string, and *v* does not have a constant value of `null`, then the pattern is considered matching if `System.MemoryExtensions.SequenceEqual<char>(e, System.MemoryExtensions.AsSpan(v))` returns `true`; otherwise
 - the pattern `P` *matches* the value *e* if `object.Equals(e, v)` returns `true`.
 
 > *Example*: The `switch` statement in the following method uses five constant patterns in its case labels.

standard/standard-library.md

@@ -404,6 +404,15 @@ namespace System
 
     public abstract class FormattableString : IFormattable { }
 
+    public static class MemoryExtensions
+    {
+        public static ReadOnlySpan<char> AsSpan (this string? text);
+        public static bool SequenceEqual<T> (this Span<T> span, ReadOnlySpan<T> other)
+          where T : IEquatable<T>;
+        public static bool SequenceEqual<T> (this ReadOnlySpan<T> span,
+          ReadOnlySpan<T> other) where T : IEquatable<T>;
+    }
+
     public class OperationCanceledException : Exception
     {
         public OperationCanceledException();
@@ -1383,6 +1392,7 @@ The following library types are referenced in this specification. The full names
 - `global::System.IntPtr`
 - `global::System.InvalidCastException`
 - `global::System.InvalidOperationException`
+- `global::System.MemoryExtensions`
 - `global::System.NotSupportedException`
 - `global::System.Nullable<T>`
 - `global::System.NullReferenceException`