What is the correspondence symbol with a hat or wedge over equals?

Before somebody suggests MnSymbol…

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\providecommand{\hateq}{\mathrel{\mathpalette\my@hat@eq\relax}}
\newcommand{\my@hat@eq}[2]{%
  \begingroup
  \sbox\z@{$\m@th#1=$}%
  \ooalign{%
    \hidewidth\raisebox{-0.3\ht\z@}{$\m@th#1\widehat{}$}\hidewidth\cr
    \box\z@\cr
  }%
  \endgroup
}
\makeatother

\begin{document}

$A\cong B\hateq C$                           

\end{document}

With \hat instead of \widehat:

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\providecommand{\hateq}{\mathrel{\mathpalette\my@hat@eq\relax}}
\newcommand{\my@hat@eq}[2]{%
  \begingroup
  \sbox\z@{$\m@th#1=$}%
  \ooalign{%
    \hidewidth\raisebox{-0.2\ht\z@}{$\m@th#1\hat{}$}\hidewidth\cr
    \box\z@\cr
  }%
  \endgroup
}
\makeatother

\begin{document}

$A\cong B\hateq C$

\end{document}

What you want could be obtained using

\documentclass[12pt]{article}
\usepackage{amsmath}
\newcommand{\circumeq}{\mathrel{\widehat{=}}}
\begin{document}
\begin{equation*}
a \circumeq b
\end{equation*}
\end{document} 

The ≙ symbol is U+2259 in Unicode. It has the name \wedgeq in several newer packages, including unicode-math and stix. It is \hateq in two older ones, mnsymbol and fdsymbol. One completely obsolete one, boisik, has \corresponds.

Edit: Sorry, just noticed that Barbara Beeton said that in a comment first. Props!