[ENH]: Parser errors should mention that commands do not exist

[mathstuf]

Problem

I recently hit a parser error after upgrading matplotlib from 3.4.2 to 3.7.2 in our build:

\sum_{i=0}^\infty x_i * \frac{3}{4} \binom{3}{4} \stackrel{3}{4} * \sqrt[3]{x}
                                                 ^
ParseFatalException: Expected token, found '\'  (at char 49), (line:1, col:50)

After digging through a few rabbit holes (e.g., reverting from pyparsing 3.1.x to 3.0.9), it turns out that stackrel had been deprecated all the way back in 3.1.0.

Proposed solution

It would be nice if, instead of an error like this, it says something like 'stackrel is not a recognized command". Even better would be to have the stackrel deprecation bring up a warning/error at this point.

[ksunden]

This is a result of a change to pyparsing that I'm pretty sure we have largely fixed, though it did incur changes in both mpl and pyparsing.

The happy path always worked, just there are some combinations that get less helpful error messages like this.

On main I get:

ParseFatalException: Unknown symbol: \stackrel, found '\'  (at char 49), (line:1, col:50)

xref:

• MNT: Unpin pyparsing, xfail error message tests for pyparsing 3.1.0 #26431
• Limit Forward references in Mathtext parser #26198
• Backport PR #26431 on branch v3.7.x (MNT: Unpin pyparsing, xfail error message tests for pyparsing 3.1.0) #26432 (Backport to the 3.7 release branch that needed commits from both the above)
• Limit error message overriding pyparsing/pyparsing#493

This will be fixed with 3.7.3 (when released), though also requires pyparsing !=3.1.0 for the error message. We decided not to enforce that restriction for install since working code continued to work, but did add some xfails to our test suite with the affected combinations.

Since this is fixed on main/the release branch, I will close this

[mathstuf]

Thanks for the info. Can this get a 3.7.3 milestone too so that it gets tracked for others searching?