[mcs] Fix null coalescing operator on cast of source expression resol…

…ved to implicit user operator conversion. Fixes #36161

mcs/mcs/nullable.cs

@@ -1073,6 +1073,7 @@ public class NullCoalescingOperator : Expression
 	{
 		Expression left, right;
 		Unwrap unwrap;
+		bool user_conversion_left;
 
 		public NullCoalescingOperator (Expression left, Expression right)
 		{
@@ -1220,6 +1221,7 @@ Expression ConvertExpression (ResolveContext ec)
 				return ReducedExpression.Create (right, this, false).Resolve (ec);
 
 			left = Convert.ImplicitConversion (ec, unwrap ?? left, rtype, loc);
+			user_conversion_left = left is UserCast;
 			type = rtype;
 			return this;
 		}
@@ -1283,10 +1285,14 @@ public override void Emit (EmitContext ec)
 			// Null check is done on original expression not after expression is converted to
 			// result type. This is in most cases same but when user conversion is involved
 			// we can end up in situation when user operator does the null handling which is
-			// not what the operator is supposed to do
+			// not what the operator is supposed to do.
+			// There is tricky case where cast of left expression is meant to be cast of
+			// whole source expression (null check is done on it) and cast from right-to-left
+			// conversion needs to do null check on unconverted source expression.
 			//
-			var op_expr = left as UserCast;
-			if (op_expr != null) {
+			if (user_conversion_left) {
+				var op_expr = (UserCast) left;
+
 				op_expr.Source.Emit (ec);
 				LocalTemporary temp;
 

mcs/tests/test-931.cs

@@ -0,0 +1,20 @@
+using System;
+
+class MainClass
+{
+	public static implicit operator string (MainClass src)
+	{
+		return null;
+	}
+
+	public static int Main ()
+	{
+		var obj = new MainClass ();
+		var s = "x";
+		var res = (string) obj ?? s;
+		if (res != "x")
+			return 1;
+
+		return 0;
+	}
+}

mcs/tests/ver-il-net_4_x.xml

@@ -51840,6 +51840,19 @@
       </method>
     </type>
   </test>
+  <test name="test-931.cs">
+    <type name="MainClass">
+      <method name="System.String op_Implicit(MainClass)" attrs="2198">
+        <size>10</size>
+      </method>
+      <method name="Int32 Main()" attrs="150">
+        <size>60</size>
+      </method>
+      <method name="Void .ctor()" attrs="6278">
+        <size>7</size>
+      </method>
+    </type>
+  </test>
   <test name="test-94.cs">
     <type name="Base">
       <method name="Int32 IVehicle.Start()" attrs="481">