Align mcs's tuple deconstruction behavior with csc

Before the code
(a, b) = (b, a);
was the equivalent of
a = b;
b = a;

now, it evaluates it to so something like
tmp1 = a;
tmp2 = b;
a = tmp2;
b = tmp1;

Signed-off-by: Bijan Tabatabai <bijan311@yahoo.com>

mcs/mcs/tuples.cs

@@ -447,6 +447,7 @@ class TupleDeconstruct : ExpressionStatement, IAssignMethod
 	{
 		Expression source;
 		List<Expression> targetExprs;
+		List<Expression> tempExprs;
 		List<BlockVariable> variables;
 		Expression instance;
 
@@ -455,13 +456,17 @@ public TupleDeconstruct (List<Expression> targetExprs, Expression source, Locati
 			this.source = source;
 			this.targetExprs = targetExprs;
 			this.loc = loc;
+
+			tempExprs = new List<Expression> ();
 		}
 
 		public TupleDeconstruct (List<BlockVariable> variables, Expression source, Location loc)
 		{
 			this.source = source;
 			this.variables = variables;
 			this.loc = loc;
+
+			tempExprs = new List<Expression> ();
 		}
 
 		public override Expression CreateExpressionTree (ResolveContext ec)
@@ -507,6 +512,15 @@ protected override Expression DoResolve (ResolveContext rc)
 					instance = expr_variable.CreateReferenceExpression (rc, loc);
 				}
 
+				var element_srcs = new List<Expression> ();
+				var src_names = new List<string> ();
+				for (int i = 0; i < target_count; ++i) {
+					var element_src = tupleLiteral == null ? new MemberAccess (instance, NamedTupleSpec.GetElementPropertyName (i)) : tupleLiteral.Elements [i].Expr;
+					element_srcs.Add (element_src);
+					if (element_src is VariableReference)
+						src_names.Add ((element_src as VariableReference)?.Name);
+				}
+
 				for (int i = 0; i < target_count; ++i) {
 					var tle = src_type.TypeArguments [i];
 
@@ -537,8 +551,17 @@ protected override Expression DoResolve (ResolveContext rc)
 						variable.PrepareAssignmentAnalysis ((BlockContext)rc);
 					}
 
-					var element_src = tupleLiteral == null ? new MemberAccess (instance, NamedTupleSpec.GetElementPropertyName (i)) : tupleLiteral.Elements [i].Expr;
-					targetExprs [i] = new SimpleAssign (targetExprs [i], element_src).Resolve (rc);
+					var element_target = (targetExprs [i] as SimpleName)?.LookupNameExpression (rc, MemberLookupRestrictions.None);
+
+					if (element_target != null && src_names.Contains ((element_target as VariableReference)?.Name)) {
+						var tempType = element_target.Resolve (rc).Type;
+
+						var temp = new LocalTemporary (tempType);
+						tempExprs.Add (new SimpleAssign (temp, element_srcs [i]).Resolve (rc));
+						targetExprs [i] = new SimpleAssign (targetExprs [i], temp).Resolve (rc);
+					} else {
+						targetExprs [i] = new SimpleAssign (targetExprs [i], element_srcs [i]).Resolve (rc);
+					}
 				}
 
 				eclass = ExprClass.Value;
@@ -572,9 +595,24 @@ public override void Emit (EmitContext ec)
 			if (instance != null)
 				((ExpressionStatement)source).EmitStatement (ec);
 
-			foreach (ExpressionStatement expr in targetExprs)
+			foreach (ExpressionStatement expr in tempExprs) {
+				var temp = (expr as Assign)?.Target as LocalTemporary;
+				if (temp == null)
+					continue;
+
+				temp.AddressOf (ec, AddressOp.LoadStore);
+				ec.Emit (OpCodes.Initobj, temp.Type);
+				expr.Emit (ec);
+			}
+
+			foreach (ExpressionStatement expr in targetExprs) {
 				expr.Emit (ec);
 
+				var temp = (expr as Assign)?.Source as LocalTemporary;
+				if (temp != null)
+					temp.Release (ec);
+			}
+
 			var ctor = MemberCache.FindMember (type, MemberFilter.Constructor (null), BindingRestriction.DeclaredOnly | BindingRestriction.InstanceOnly) as MethodSpec;
 			ec.Emit (OpCodes.Newobj, ctor);
 		}
@@ -589,9 +627,24 @@ public override void EmitStatement (EmitContext ec)
 
 			if (instance != null)
 				((ExpressionStatement) source).EmitStatement (ec);
-
-			foreach (ExpressionStatement expr in targetExprs)
+
+			foreach (ExpressionStatement expr in tempExprs) {
+				var temp = (expr as Assign)?.Target as LocalTemporary;
+				if (temp == null)
+					continue;
+
+				temp.AddressOf (ec, AddressOp.LoadStore);
+				ec.Emit (OpCodes.Initobj, temp.Type);
+				expr.EmitStatement (ec);
+			}
+
+			foreach (ExpressionStatement expr in targetExprs) {
 				expr.EmitStatement (ec);
+
+				var temp = (expr as Assign)?.Source as LocalTemporary;
+				if (temp != null)
+					temp.Release (ec);
+			}
 		}
 
 		public void Emit (EmitContext ec, bool leave_copy)

mcs/tests/test-tuple-11.cs

@@ -0,0 +1,20 @@
+using System;
+
+class Program
+{
+	public static int Main ()
+	{
+		int x = 1;
+		int y = 2;
+
+		(x, y) = (y, x);
+
+		if (x != 2)
+			return 1;
+
+		if (y != 1)
+			return 2;
+
+		return 0;
+	}
+}

mcs/tests/ver-il-net_4_x.xml

@@ -74502,6 +74502,16 @@
       </method>
     </type>
   </test>
+  <test name="test-tuple-11.cs">
+    <type name="Program">
+	  <method name="Int32 Main()" attrs="150">
+	    <size>70</size>
+	  </method>
+	  <method name="Void .ctor()" attrs="6278">
+        <size>7</size>
+	  </method>
+	</type>
+  </test>
   <test name="test-var-01.cs">
     <type name="Test">
       <method name="Int32 Main()" attrs="150">