\big does not force horizontal mode

[blefloch]

Brief outline of the bug

Using \big at the beginning of a paragraph has the same problem as \hbox: the parenthesis ends up along in a line.

Minimal example showing the bug

\RequirePackage{latexbug}
\documentclass{article}
\usepackage{amsmath}
\begin{document}
We claim that $16^{3^2} = (8^4)^3$.

\big(Recall that $x^{y^z}=x^{(y^z)}$.\big)
\end{document}

Log file (required) and possibly PDF file

test.log
test.pdf

[blefloch]

This problem does not need amsmath.

[davidcarlisle]

comments and possible code changes for this issue are combined with those for issue #49

[eg9]

Neither does \smash, for instance, nor \, and \enspace. Only \, is mentioned in the LaTeX manual, which doesn't tell about \big in text mode.

[kberry]

also \thinspace and \negthinspace (ltspace.dtx)

[FrankMittelbach]

My guess is that for all the spaces the 2.09 rule "save tokens" was applied as starting a paragraph with a space is rather uncommon and back then each token did count. As this is no longer true these days one should add \leavevmode to all of them in my opinion to support even the rare cases as right now they are simply unusable at that point.