documentclass{minimal}
usepackage{tikz}
begin{document}
pgfversionpar
begin{tikzpicture}
draw[blue] plot[domain=-1:1] (x,{x^2 + x^3});
draw[xshift=4cm,blue] plot[domain=-1:1] (x,{(x)^2 + (x)^3});
end{tikzpicture}
end{document}

documentclass{minimal}
usepackage{tikz}
makeatletter
deftikz@scan@no@calculator#1(#2){%
    patch@tikz(#2)%
    expandaftertikz@@scan@@no@calculatorexpandafter#1tikz@temp
    }
defpatch@tikz(#1,#2){%
    {expandafterletexpandaftersave@tikz@plot@vartikz@plot@var
     expandafterdeftikz@plot@var{(save@tikz@plot@var)}%
     xdeftikz@temp@i{#1}%
     xdeftikz@temp@ii{#2}%
     }%
    edeftikz@temp{({tikz@temp@i},{tikz@temp@ii})}%
    }
makeatother
begin{document}
pgfversionpar
begin{tikzpicture}
draw[blue,variable=t] plot[domain=-1:1] (t,{t^2 + t^3});
draw[xshift=4cm,blue] plot[domain=-1:1] (x,{(x)^2 + (x)^3});
draw[xshift=8cm,scale=0.5,domain=-3.141:3.141,smooth,variable=t]
plot ({t*sin(t r)},{t*cos(t r)});
end{tikzpicture}
end{document}

documentclass{minimal}
usepackage{pgf}
begin{document}
pgfversionpar
pgfmathparse{-1^2}pgfmathresult
end{document}

draw plot[domain=-1:1] (x,-x^2);

draw plot[domain=-1:1] (x,{-(x)^2});

draw plot[domain=-1:1] (x,{-x^2});

draw plot[domain=-1:1] (x,0-x^2);