Add extended-property-patterns.md (#4585)

proposals/extended-property-patterns.md

@@ -0,0 +1,51 @@
+# Extended property patterns
+
+## Summary
+[summary]: #summary
+
+Allow property subpatterns to reference nested members, for instance:
+```cs
+if (e is MethodCallExpression { Method.Name: "MethodName" })
+``` 
+Instead of:
+```cs
+if (e is MethodCallExpression { Method: { Name: "MethodName" } })
+```
+
+## Motivation
+[motivation]: #motivation
+
+When you want to match a child property, nesting another recursive pattern adds too much noise which will hurt readability with no real advantage.
+
+## Detailed design
+[design]: #detailed-design
+
+The [*property_pattern*](https://github.com/dotnet/csharplang/blob/main/proposals/csharp-8.0/patterns.md#property-pattern) syntax is modified as follow:
+
+```diff
+property_pattern
+  : type? property_pattern_clause simple_designation?
+  ;
+
+property_pattern_clause
+  : '{' (subpattern (',' subpattern)* ','?)? '}'
+  ;
+
+subpattern
+- : identifier ':' pattern
++ : subpattern_name ':' pattern
+  ;
+
++subpattern_name
++ : identifier 
++ | subpattern_name '.' identifier
++ ;
+```
+
+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: { Prop1: pattern } }`.
+
+Note that 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`.
+
+Repeated member paths are allowed. Under the hood, such member accesses are simplified to be evaluated once.