engine: a maths macro's optional argument - #166
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\newcommand{\cmd}[n][default] makes the FIRST of the n parameters optional: \cmd is
\@Protected@testopt, which supplies {default} when no [ follows (latex.ltx:1187-1199,
\@xargdef), and the remaining n-1 are grabbed as usual. The token path already does
this (engine.go's optArg/optDefault); the maths source path read all n as MANDATORY.
That is worse than not expanding at all. With the optional argument written out the
macro still "matched", by taking the brackets themselves as arguments:
\qbin [p]{N}{k} -> \genfrac []{0pt}{}{p}{]}_{[}{N}{k}
\qbin {N}{k}+1 -> \genfrac []{0pt}{}{k}{+}_{N}1
so the formula came out silently wrong, the + of an expression becoming an argument.
Without it, nothing matched and the formula was dropped — which is how a paper's own
\newcommand{\qbinom}[3][q]{\genfrac[]{0pt}{}{#2}{#3}_{#1}} lost 12 equations.
Measured over 200 arXiv papers and 200 beamer talks: 5 documents change for +1234
glyphs. One repaginates, 362 -> 363 against tectonic's 333 — a document already 29
pages long of it — and the page error against the 157 renders moves 669 -> 670.
beamer does not move by one glyph.
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\newcommand{\cmd}[n][default]makes the first of the n parameters optional:\cmdis\@protected@testopt, which supplies{default}when no[follows (latex.ltx:1187-1199,\@xargdef), and the remaining n−1 are grabbed as usual. The token path already does this (optArg/optDefaultin engine.go); the maths source path read all n as mandatory.That is worse than not expanding at all. With the optional argument written out the macro still "matched", by taking the brackets themselves as arguments:
— the formula came out silently wrong, the
+of an expression becoming an argument. Without the optional argument nothing matched and the formula was dropped instead, which is how a paper's ownlost 12 equations.
Measured — 200 arXiv papers, 200 beamer talks
5 documents change for +1234 glyphs. One repaginates, 362 → 363 against tectonic's 333 — a document already 29 pages clear of it — and the page error against the 157 renders moves 669 → 670. beamer does not move by one glyph.
Test
mathoptarg_test.goasserts the expansions themselves, since that is where the corruption is visible; all three fail onmainwith the stolen-bracket strings above. A second test takes it end to end: the formula must reach the page with the text around it.