Fix #3237: Use ref readonly locals for readonly.ldelema

ICSharpCode.Decompiler/IL/Transforms/ILInlining.cs

@@ -537,6 +537,9 @@ internal static ExpressionClassification ClassifyExpression(ILInstruction inst)
 			}
 		}
 
+		/// <summary>
+		/// Gets whether the ILInstruction will turn into a C# expresion that is considered readonly by the C# compiler.
+		/// </summary>
 		internal static bool IsReadonlyReference(ILInstruction addr)
 		{
 			switch (addr)

ICSharpCode.Decompiler/IL/Transforms/IntroduceRefReadOnlyModifierOnLocals.cs

@@ -52,8 +52,18 @@ bool IsUsedAsRefReadonly(ILVariable variable)
 		{
 			foreach (var store in variable.StoreInstructions.OfType<StLoc>())
 			{
+				// Check if C# requires that the local is ref-readonly in order to allow the store:
 				if (ILInlining.IsReadonlyReference(store.Value))
 					return true;
+				// Check whether the local needs to be ref-readonly to avoid changing the semantics of
+				// a readonly.ldelema:
+				ILInstruction val = store.Value;
+				while (val is LdFlda ldflda)
+				{
+					val = ldflda.Target;
+				}
+				if (val is LdElema { IsReadOnly: true })
+					return true;
 			}
 			return false;
 		}