Add pattern desugaring visitor

SyntaxVisitors/SugarVisitors/IsPatternVisitor.cs

@@ -0,0 +1,41 @@
+using PascalABCCompiler.SyntaxTree;
+using PascalABCCompiler.TreeConversion;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace SyntaxVisitors.SugarVisitors
+{
+    public class IsPatternVisitor : BaseChangeVisitor
+    {
+        public static IsPatternVisitor New => new IsPatternVisitor();
+
+        public override void visit(is_pattern_expr isPatternExpr)
+        {
+            if (isPatternExpr.right is type_pattern)
+                DesugarTypePattern(isPatternExpr);
+        }
+
+        void DesugarTypePattern(is_pattern_expr isPatternExpr)
+        {
+            // Замена is_pattern на вызов вспомогательной функции PABCSystem.IsTest 
+            expression expression = isPatternExpr.left;
+            type_pattern pattern = isPatternExpr.right as type_pattern;
+            var isTestFunc = SubtreeCreator.CreateSystemFunctionCall("IsTest", expression, pattern.identifier);
+            ReplaceUsingParent(isPatternExpr, isTestFunc);
+
+            // Объявление переменной в ближайшем statement_list
+            for (int i = listNodes.Count - 1; i >= 0; i--)
+            {
+                var statements = listNodes[i] as statement_list;
+                if (statements != null)
+                {
+                    statements.InsertBefore(
+                        listNodes[i + 1] as statement, 
+                        new var_statement(pattern.identifier, pattern.type));
+                }
+            }
+        }
+    }
+}

TestSuite/CompilationSamples/PABCSystem.pas

@@ -1691,6 +1691,11 @@ function Dict<TKey, TVal>(params pairs: array of KeyValuePair<TKey, TVal>): Dict
 /// Возвращает пару элементов (ключ, значение)
 function KV<TKey, TVal>(key: TKey; value: TVal): KeyValuePair<TKey, TVal>;
 
+// -----------------------------------------------------
+//>>     Вспомогательные функции для pattern matching
+// -----------------------------------------------------
+function IsTest<T>(obj: object; var res: T): boolean;
+
 //{{{--doc: Конец секции интерфейса для документации }}} 
 
 
@@ -4180,6 +4185,21 @@ begin
   Result := new KeyValuePair<TKey, TVal>(key, value);
 end;
 
+
+function IsTest<T>(obj: object; var res: T): boolean;
+begin
+  if obj is T then
+    begin
+      res := T(obj);
+      Result := true;
+    end
+  else
+    begin
+      res := default(T);
+      Result := false;
+    end;
+end;
+
 {function read_lexem: string; 
 var
   c: char;