bug#24896: JSX prop indentation after fat arrow

[Felipe Ochoa]

(Preemptive apologies if this is the wrong list/format for this comment -- first time filer here!)

When indenting JSX code using js2- or js-mode, the indentation function gets confused when there's a fat arrow function in a JSX prop. Compare the way the following two code blocks are auto-indented:

const Component = props => ( // Incorrect indentation
    <FatArrow a={e => c}
      b={123}>
    </FatArrow>
);

const Component = props => ( // Correct indentation
    <NoFatArrow a={123}
                b={123}>
    </NoFatArrow>
);

I've tracked the problem down to `sgml-calculate-indent' using `parse-partial-sexp' with `sgml-tag-syntax-table', where `>' is treated as a close-parenthesis character (and thus the end-of-tag marker). I don't think there's a way to patch the syntax table that would let `>' flip between punctuation and close-parens based on context, but one possible fix when using js2-mode (not sure about js-mode) is to apply a "." 'syntax-table text property to the `>' when parsing a fat arrow.

Unfortunately, `js-jsx-indent-line' calls `sgml-indent-line' using `js--as-sgml', which sets `parse-sexp-lookup-properties' to nil.

Would there be any harm in setting `parse-sexp-lookup-properties' to t instead? As far as I can tell, js-mode and js2-mode only use 'syntax-table propeties for regex literals.

As a side-note, there may well be a different solution to this problem; I still don't understand why the following block is indented correctly:

const Component = props => (
    <WithRegex a={/>/}
               b={123}>
    </WithRegex>
);