newenvironment with empheq inside tcolorbox

Question

I have to define a new environment doing something like that.

\documentclass[12pt]{article}
\usepackage{tcolorbox,empheq}
\begin{document}
\begin{tcolorbox}
\begin{empheq}{align}
a=b
\end{empheq}
\end{tcolorbox}
\end{document}

This code works fine, but if I try something like testa or testb, I get errors.

\documentclass[12pt]{article}
\usepackage{tcolorbox,empheq}

\newenvironment{testa}{\tcolorbox\empheq{align}}{\endempheq\endtcolorbox}
\newenvironment{testb}{\tcolorbox\setkeys{EmphEqEnv}{align}\EmphEqMainEnv}{\endEmphEqMainEnv\endtcolorbox}

\begin{document}
\begin{testa}
a=b
\end{testa}
\end{document}

Is there a solution ?

Answer 1

I suggest to use a temporary wrapper environment for empheq and then 'wrap' \tcolorboxenvironment around it:

\documentclass[12pt]{article}
\usepackage[most]{tcolorbox}
\usepackage{empheq}

\newenvironment{testa}{%
  \empheq{align}
}{\endempheq}

\tcolorboxenvironment{testa}{colback=yellow!40!white}


\begin{document}


\begin{testa}
a=b
\end{testa}
\end{document}

Answer 2

I understand you want to combine tcolorbox with empheq. Instead of a tcolorbox which contains an empheq environment, you should declare an empheq environment with an optional parameter which defines the colorbox aspect. This combination can be declared as a newenvironment

\documentclass{article}

\usepackage{amsmath}
\usepackage{empheq}
\usepackage[most]{tcolorbox}

\newtcbox{\mymath}[1][]{%
    nobeforeafter, math upper, tcbox raise base,
    enhanced, colframe=blue!30!black,
    colback=blue!30, boxrule=1pt,
    #1}

\newenvironment{testa}[1]{%
    \empheq[box={#1}]{align}}{\endempheq}

\begin{document}
\begin{empheq}[box=\mymath]{align}
    c_i &= \langle\psi|\phi\rangle \\
    d_i &= \sqrt{x^2-1}
\end{empheq}

\begin{testa}{\mymath[colback=red!30]}
    c_i &= \langle\psi|\phi\rangle \\
    d_i &= \sqrt{x^2-1}
\end{testa}

\end{document}

Some other examples in Attractive Boxed Equations