[v4 beta 6] overline + prime causing unexpected "double exponent" error

[pkra]

Our testing ran into a lot of unexpected Prime causes double exponent: use braces to clarify processing errors after upgrading.

It seems to always come down to overline and primes, e.g.,

• \overline{\mathfrak{D}}'_j
• \overline{c}''=\overline{n}
• \begin{equation*} \|\overline{c}'\|_{\infty }\leqslant \|\overline{n}\|_{1}= \|n\|_{1}. \end{equation*}

[dpvc]

This is due to the changes from mathjax/MathJax-src/pull/1069, which left out a check after

https://github.com/mathjax/MathJax-src/blob/70ea011e8f84eeefb352e5167e843a6033c42d3a/ts/input/tex/base/BaseMethods.ts#L260

like

https://github.com/mathjax/MathJax-src/blob/70ea011e8f84eeefb352e5167e843a6033c42d3a/ts/input/tex/base/BaseMethods.ts#L161

Here is a configuration that can be used to add that in:

MathJax = {
  tex: {packages: {'[+]': ['fix-primes']}},
  startup: {
    ready() {
      const {MacroMap} = MathJax._.input.tex.TokenMap;
      const {Configuration} = MathJax._.input.tex.Configuration;
      const NodeUtil = MathJax._.input.tex.NodeUtil.default;
      const {entities} = MathJax._.util.Entities;
      new MacroMap('fix-primes', {
        '\'':  'Prime',
        '\u2019': 'Prime',
        '\u2032': 'Prime'
      }, {
        Prime(parser, c) {
          let base = parser.stack.Prev();
          if (!base) {
            base = parser.create('token', 'mi');
          }
          if ((NodeUtil.isType(base, 'msubsup') && !NodeUtil.isType(base, 'msup') &&
               NodeUtil.getChildAt(base, base.sup)) ||
              (NodeUtil.isType(base, 'munderover') && !NodeUtil.isType(base, 'mover') &&
               NodeUtil.getChildAt(base, base.over) &&
               !NodeUtil.getProperty(base, 'subsupOK'))) {
            throw new TexError('DoubleExponentPrime', 'Prime causes double exponent: use braces to clarify');
          }
          let sup = '';
          parser.i--;
          do {
            sup += entities.prime; parser.i++, c = parser.GetNext();
          } while (c === '\'' || c === entities.rsquo || c === entities.prime);
          sup = ['', '\u2032', '\u2033', '\u2034', '\u2057'][sup.length] || sup;
          const node = parser.create('token', 'mo', {variantForm: true}, sup);
          parser.Push(parser.itemFactory.create('prime', base, node));
        }
      });
      Configuration.create('fix-primes', {
        handler: {character: ['fix-primes']}
      });
      MathJax.startup.defaultReady();
    }
  }
};

I will make a PR to add the missing check.