operatorname and mathbin spacing

Question

It seems like operators (e.g. \sin) don't mix nicely with binary operations (e.g. \circ). An example:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

$\sin \circ f$

$\mathrm{sin} \circ f$

$\operatorname{thing} \mathbin{a} f$

$\mathrm{thing} \mathbin{a} f$

\end{document}

I want the spacing for the \operatorname to be as it is for the mathrm (of course I don't want to actually change the operatorname to a mathrm). How can I achieve this?

Answer 1

The problem is the same as in Underbrace changing spacing of operators, because \underbrace creates an Op atom just like \sin.

The solution is to use

${\sin}\circ f$

in this case, because the braces make a subformula, which is considered as an ordinary atom.