LaTeX math tabbing: Can we make alignat work this way?

Question

The second alignat below illustrates my problem.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{alignat*}{2}
  a &= & b \\
    &  & + c
\end{alignat*}
\begin{alignat*}{2}
  a &= & b \\
    &  & + c \\
    &= \text{long expression}
\end{alignat*}
\end{document}

I want the third line of the equation not to alter spacing on previous lines. In other words, I want a math environment that behaves similarly to the tabbing environment. Could somebody help?

Edit: The example I show above is a simplified one. In my real problem, the part corresponding to = b is quite complicated. If we use Werner's solution below, we would be repeating the whole complicated expression in \phantom{}. So, I guess there is no ready-made tabbing-like environment?

My current workaround is to use \hspace*{} in place of \phantom{}, manually adjusting the position. (I'm not claiming it is better. Manual adjustment is annoying.)

Edit 2: Werner has made me realize that I simplified my code so much that a simple \phantom{{}={}} is a good solution to the simplified code. So, the example below is a more accurate illustration of my original problem:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\begin{align*}
  a &= b [ c + d \\
    &\phantom{{}=b [ c} + e] \\
    &= \text{long expression}
\end{align*}
\end{document}

In this case, (I think) we have to repeat part of the previous line in \phantom, which is annoying when b and c are complicated. As I said in my "Edit" above, I'm currently using \hspace* to avoid repeating the complicated expression in a phantom.

Answer 1

There are a myriad of ways you can achieve this alignment. Here are just two:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\begin{alignat*}{2}
  a &={} && b \\
    &    && + c
\end{alignat*}

\begin{align*}
  a &= b \\
    &\phantom{{}={}} + c \\
    &= \text{long expression}
\end{align*}
\end{document}

Answer 2

To get the third line to not affect the spacing of the first two you can use \rlap{\text{long expression}} and thus no need to change anything before that line:

Note:

• I also added a curly brace pair as in the {}+c so that the + is treated as a binary operator.

Code:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{alignat*}{2}
  a &= & b \\
    &  & {}+ c
\end{alignat*}
\begin{alignat*}{2}
  a &= & b \\
    &  & {}+ c \\
    &= \rlap{\text{long expression}}
\end{alignat*}
\end{document}