Fix #2321: Missing readonly modifier for ref local

ICSharpCode.Decompiler.Tests/TestCases/Pretty/RefLocalsAndReturns.cs

@@ -137,6 +137,13 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 				Console.WriteLine("No inlining");
 				Console.WriteLine(field.GetHashCode());
 			}
+
+			public void RefReadonlyCallVirt(RefLocalsAndReturns provider)
+			{
+				ref readonly NormalStruct readonlyRefInstance = ref provider.GetReadonlyRefInstance<NormalStruct>();
+				Console.WriteLine("No inlining");
+				readonlyRefInstance.Method();
+			}
 		}
 
 		private static int[] numbers = new int[10] { 1, 3, 7, 15, 31, 63, 127, 255, 511, 1023 };
@@ -156,6 +163,11 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			throw new NotImplementedException();
 		}
 
+		public ref readonly T GetReadonlyRefInstance<T>()
+		{
+			throw new NotImplementedException();
+		}
+
 		public void CallOnRefReturn()
 		{
 			// Both direct calls:

ICSharpCode.Decompiler/IL/Transforms/ILInlining.cs

@@ -459,6 +459,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 					return ldloc.Variable.IsRefReadOnly;
 				case Call call:
 					return call.Method.ReturnTypeIsRefReadOnly;
+				case CallVirt call:
+					return call.Method.ReturnTypeIsRefReadOnly;
 				case CallIndirect calli:
 					return calli.FunctionPointerType.ReturnIsRefReadOnly;
 				case AddressOf _: