Fix reflection-calling Set method on arrays

src/coreclr/nativeaot/System.Private.CoreLib/src/Internal/Runtime/Augments/RuntimeAugments.cs

@@ -120,6 +120,32 @@ public static unsafe Array NewMultiDimArray(RuntimeTypeHandle typeHandleForArray
             return Array.NewMultiDimArray(typeHandleForArrayType.ToMethodTable(), pImmutableLengths, lengths.Length);
         }
 
+        public static unsafe void SetArrayValue(Array array, int[] indices, object value)
+        {
+            MethodTable* elementMT = array.ElementMethodTable;
+
+            if (elementMT->IsPointer || elementMT->IsFunctionPointer)
+            {
+                Debug.Assert(value.GetMethodTable()->ValueTypeSize == IntPtr.Size);
+                elementMT = value.GetMethodTable();
+            }
+
+            if (elementMT->IsValueType)
+            {
+                Debug.Assert(value.GetMethodTable()->IsValueType && elementMT->ValueTypeSize == value.GetMethodTable()->ValueTypeSize);
+                nint flattenedIndex = array.GetFlattenedIndex(indices);
+                ref byte element = ref Unsafe.AddByteOffset(ref MemoryMarshal.GetArrayDataReference(array), (nuint)flattenedIndex * array.ElementSize);
+                RuntimeImports.RhUnbox(value, ref element, elementMT);
+            }
+            else
+            {
+                RuntimeImports.RhCheckArrayStore(array, value);
+                nint flattenedIndex = array.GetFlattenedIndex(indices);
+                ref object element = ref Unsafe.Add(ref Unsafe.As<byte, object>(ref MemoryMarshal.GetArrayDataReference(array)), flattenedIndex);
+                element = value;
+            }
+        }
+
         public static IntPtr GetAllocateObjectHelperForType(RuntimeTypeHandle type)
         {
             return RuntimeImports.RhGetRuntimeHelperForType(type.ToMethodTable(), RuntimeHelperKind.AllocateObject);

src/coreclr/nativeaot/System.Private.CoreLib/src/System/Array.NativeAot.cs

@@ -727,7 +727,7 @@ private unsafe nint GetFlattenedIndex(int rawIndex)
             return rawIndex;
         }
 
-        private unsafe nint GetFlattenedIndex(ReadOnlySpan<int> indices)
+        internal unsafe nint GetFlattenedIndex(ReadOnlySpan<int> indices)
         {
             // Checked by the caller
             Debug.Assert(indices.Length == Rank);

src/coreclr/nativeaot/System.Private.CoreLib/src/System/Reflection/Runtime/TypeInfos/RuntimeArrayTypeInfo.cs

@@ -213,7 +213,7 @@ internal sealed override IEnumerable<RuntimeMethodInfo> SyntheticMethods
                             for (int i = 0; i < rank; i++)
                                 indices[i] = (int)(args[i]);
                             object value = args[rank];
-                            array.SetValue(value, indices);
+                            RuntimeAugments.SetArrayValue(array, indices, value);
                             return null;
                         }
                     );