Warning in version 11.0 when loading package Combinatorica

Question

After the usual and well-known warning

General::compat: "Combinatorica Graph and Permutations functionality has been superseded 
by preloaded functionality..."

that was present in version 10, a new warning appears:

SetDelayed::write: "Tag Element in a_List \[Element] {index___} is Protected."

This seems to be potentially serious.

Which statement in Combinatorica causes it, and what are the consequences?

Edit 2016/09/10: Found it.

Combinatorica contains a function Element:

Element[a_List, {index___}] := a[[index]]    

The function Element in the environment System` has the following added Information:

Element[patt, dom] asserts that any expression matching the pattern patt is an element of dom. The use of the function Element in Combinatorica is now obsolete, though the function call Element[a, p] still gives the pth element of nested list a, where p is a list of indices.

So why do some users not get the warning/error but I do?

Answer 1

When the first message is generated, the message formatting code is autoloaded from .mx files. This may cause the attributes of certain symbols to be reset, or rather replaced with what has been stored in one of those .mx files.

A minimal example that can also be run in a standalone kernel

In[1]:= Unprotect[Element]; Attributes[Element]

Out[1]= {}

In[2]:= IntervalSlider; 

In[3]:= Attributes[Element]

Out[3]= {Protected}

where the simple mention of IntervalSlider causes the NotebookTools/Controls.mx file to be autoloaded.

For a workaround of the Element problem, it is sufficient to Quiet the message:

 Quiet[Needs["Combinatorica`"], General::compat]

or trigger the autoloading before attempting to load the package:

 IntervalSlider; Needs["Combinatorica`"]

This is also the most likely reason why some users did not see the error: perhaps they already did something in their session (as simple as generating an error message) that triggered the autoload in advance of loading Combinatorica .