[Version 10.0] Feature support for extended property patterns

standard/patterns.md

@@ -211,7 +211,11 @@
     ;
 subpattern
     : pattern
-    | identifier ':' pattern
+    | subpattern_name ':' pattern
+    ;
+subpattern_name
+    : identifier
+    | subpattern_name '.' identifier
     ;
 ```
 
@@ -319,6 +323,12 @@
 
 The *property_pattern* may be used to pattern-match with anonymous types.
 
+A *property_subpattern* may reference a nested member. In such a case, the receiver for each name lookup is the type of the previous member *T₀*, starting from the *input type* of the *property_pattern*. If *T* is a nullable type, *T₀* is its underlying type, otherwise *T₀* is equal to *T*. For example, a pattern of the form `{ Prop1.Prop2: pattern }` is exactly equivalent to `{ Prop1: { Prop2: pattern } }`.
+
+> *Note*: This will include the null check when *T* is a nullable value type or a reference type. This null check means that the nested properties available will be the properties of *T₀*, not of *T*. As repeated member paths are allowed, the compilation of pattern matching can take advantage of common parts of patterns. *end note*
+<!-- markdownlint-disable MD028 -->
+
+<!-- markdownlint-enable MD028 -->
 > *Example*:
 >
 > <!-- Example: {template:"standalone-console", name:"PropertyPattern2", replaceEllipsis:true, customEllipsisReplacements: ["new object()", ";"], ignoredWarnings:["CS0642"]} -->
@@ -336,14 +346,14 @@
 > <!-- Example: {template:"standalone-console", name:"PropertyPattern3", inferOutput:true} -->
 > ```csharp
 > Console.WriteLine(TakeFive("Hello, world!"));  // output: Hello
 > Console.WriteLine(TakeFive("Hi!"));            // output: Hi!
 > Console.WriteLine(TakeFive(new[] { '1', '2', '3', '4', '5', '6', '7' }));  // output: 12345
 > Console.WriteLine(TakeFive(new[] { 'a', 'b', 'c' }));  // output: abc
 > 
 > static string TakeFive(object input) => input switch
 > {
 >     string { Length: >= 5 } s => s.Substring(0, 5),
 >     string s => s,
 >     ICollection<char> { Count: >= 5 } symbols => new string(symbols.Take(5).ToArray()),
 >     ICollection<char> symbols => new string(symbols.ToArray()),
 >     null => throw new ArgumentNullException(nameof(input)),