[mcs] Support implicit user conversion from nullable type without unwrapping. Fixes #60900 (case 970493)

Author: marek-safar

mcs/errors/cs0151-4.cs

@@ -1,5 +1,6 @@
 // CS0151: A switch expression of type `S1?' cannot be converted to an integral type, bool, char, string, enum or nullable type
-// Line: 24
+// Line: 25
+// Compiler options: -langversion:5
 
 using System;
 

mcs/errors/known-issues-net_4_x

@@ -14,6 +14,9 @@
 # Parser problems
 cs0080.cs 
 
+# Undocumented switch governing rules
+cs0151-4.cs NO ERROR
+
 # Operators
 cs0457-2.cs
 cs0457.cs

mcs/mcs/convert.cs

@@ -1232,6 +1232,13 @@ public static Expression UserDefinedConversion (ResolveContext rc, Expression so
 					FindApplicableUserDefinedConversionOperators (rc, operators, source_type_expr, target_type, restr, ref candidates);
 				}
 
+				if (source_type_expr == source && source_type.IsNullableType) {
+					operators = MemberCache.GetUserOperator (source_type.TypeArguments [0], Operator.OpType.Implicit, declared_only);
+					if (operators != null) {
+						FindApplicableUserDefinedConversionOperators (rc, operators, source_type_expr, target_type, restr, ref candidates);
+					}
+				}
+
 				if (!implicitOnly) {
 					operators = MemberCache.GetUserOperator (source_type, Operator.OpType.Explicit, declared_only);
 					if (operators != null) {

mcs/tests/gtest-647.cs

@@ -0,0 +1,34 @@
+using System;
+
+public class Program
+{
+	public static int Main ()
+	{
+		int B = default (MyStruct?); 
+		if (MyStruct.counter != 1)
+			return 1;
+
+		switch (default (MyStruct?)) {
+			case 0:
+				break;
+			default:
+				return 2;
+		}
+
+		if (MyStruct.counter != 2)
+			return 4;
+
+		return 0;
+	}
+
+	public struct MyStruct
+	{
+		public static int counter;
+
+		public static implicit operator int (MyStruct? s)
+		{
+			++counter;
+			return 0;
+		}
+	}
+}

mcs/tests/ver-il-net_4_x.xml

@@ -11064,7 +11064,7 @@
     </type>
     <type name="X">
       <method name="Int32 Beer(System.Nullable`1[IrishPub])" attrs="145">
-        <size>72</size>
+        <size>60</size>
       </method>
       <method name="Int32 Test(System.Nullable`1[System.Int32])" attrs="145">
         <size>62</size>
@@ -19505,7 +19505,7 @@
     </type>
     <type name="C">
       <method name="Int32 Main()" attrs="150">
-        <size>267</size>
+        <size>255</size>
       </method>
       <method name="Void .ctor()" attrs="6278">
         <size>7</size>
@@ -20144,6 +20144,21 @@
       </method>
     </type>
   </test>
+  <test name="gtest-647.cs">
+    <type name="Program">
+      <method name="Int32 Main()" attrs="150">
+        <size>99</size>
+      </method>
+      <method name="Void .ctor()" attrs="6278">
+        <size>7</size>
+      </method>
+    </type>
+    <type name="Program+MyStruct">
+      <method name="Int32 op_Implicit(System.Nullable`1[Program+MyStruct])" attrs="2198">
+        <size>22</size>
+      </method>
+    </type>
+  </test>
   <test name="gtest-anontype-01.cs">
     <type name="Test">
       <method name="Int32 Main()" attrs="150">