align 环境中 \isempty 与 ampersand 之间的交互

Question

\documentclass{article}
\usepackage{xparse,amsmath,xifthen}

\NewDocumentCommand{\funcA}{m}{%
    \begin{aligned}#1\end{aligned}
}
\NewDocumentCommand{\funcB}{m}{%
    \ifthenelse{1=1}{}{\begin{aligned}#1\end{aligned}}
}
\NewDocumentCommand{\funcC}{m}{%
    \ifthenelse{\isempty{#1}}{}{\begin{aligned}#1\end{aligned}}
}

\begin{document}
    \begin{align*}
        \funcA{a} % works
        \funcA{&a} % works
        \funcB{a} % works
        \funcB{&a} % works
        \funcC{a} % works
        \funcC{&a} % doesn't work
    \end{align*}
\end{document}

Answer 1

我不确定目的到底是什么。无论如何

\documentclass{article}
\usepackage{amsmath}
%\usepackage{xparse} % not needed for LaTeX 2020-10-01 or later

\ExplSyntaxOn

\NewDocumentCommand{\func}{O{c}m}
 {
  \tl_if_blank:nF { #2 } { \begin{aligned}[#1]#2\end{aligned} }
 }

\ExplSyntaxOff

\begin{document}

\begin{align*}
x+y&= \func{a} % works
\\
x+y&= \func{x&=a\\y&=b} % works
\\
x+y&= \func[t]{x&=a\\y&=b} % works
\\
x+y&= \func{} % works
\\
x+y&= \func{ } % works
\end{align*}

\end{document}

Answer 2

OP 回答了自己的问题。由 @egreg 的评论提示并使用这个帖子，我发现这有效。

\NewDocumentCommand{\funcD}{m}{%
    \ifx{#1\@empty}{}\else\begin{aligned}#1\end{aligned}\fi
}