I put \pgfresetboundingbox after \clip, what should the bounding box be?

Question

Short question

After a \clip and a \pgfresetboundingbox, the bounding box is permanently (0,0)rectangle(0,0). How can I turn on the bounding-box calculation and obtain a fairly small bounding box?

The story

The example below

\documentclass{article}
\usepackage{tikz}
\begin{document}

\tikz{
    \clip circle(10);
    \pgfresetboundingbox
    \fill circle(10);
%   \useasboundingbox(-10,-10)(10,10); % makes no difference
}

\end{document}

results in

where the rectangle is the US-letter paper from article and the center of the circle is approximately the starting point of normal documents. This shows that \pgfresetboundingbox resets the bounding box, as expected. However, a normal path does not repair it. Neither does an explicit \useasboundingbox.

The question is:

After a \clip and a \pgfresetboundingbox,
How can I turn on the calculation of bounding box?

Recall that in everyday usage, the code with \pgfresetboundingbox will always give a smaller or equal bounding box than that without \pgfresetboundingbox. In the case above, it does give me a smaller bounding box. (just too small)

Therefore I expect that if somehow we can turn the calculation on, it should not give a bounding box that is larger than that without \pgfresetboundingbox.

In other words,

After a \clip and a \pgfresetboundingbox,
How can I turn on the calculation of bounding box
such that it is the smallest but contains everything visible?

\tikz{
    \clip(5,0)--(-4,-4)--(0,5)--cycle;
    \pgfresetboundingbox
    \fill(-5,0)--(4,4)--(0,-5)--cycle;
}

should give me

where the hidden parts are

Possible application

The following code generate bad right border. (beacuse of the control point)

\tikz{
    \fill(0,-1)..controls(10,0)..(0,1);
}

It would be good if we can apply some clipping only on right border.

% Caution! imaginary code
\tikz[clip right at x=7]{
    \fill(0,-1)..controls(10,0)..(0,1);
}

which is implemented by inserting

    \clip(-\maxdimen,-\maxdimen)rectangle(7,\maxdimen);
    \pgfresetboundingbox

Remark

If two clippings are issued consecutively, the bounding box should be even smaller.

Answer 1

I see you have added some details to your question, but I had already done the first part, so I'll post it. I can see that I might have to change something for you to "reuse" the clipping, at first the question seemed to imply it was a one-time thing.

Output

Showing shapes

Hidden

Code

\documentclass{standalone}
\usepackage{tikz}

\usetikzlibrary{shapes.geometric}

\begin{document}
\tikz{
    \useasboundingbox(-10,-10)(10,10);
    \node[
        anchor=east, %fill=yellow, 
        minimum height=25cm, minimum width=5cm,
        isosceles triangle, rotate=45, 
        ] (trifill) at (current bounding box.north east) {};
        \node[
        anchor=east, %draw, dashed, 
        minimum height=25cm, minimum width=5cm,
        isosceles triangle, rotate=225
        ] (tridashed) at (current bounding box.south west) {};

\begin{scope}
    \clip (trifill.apex) foreach \i in {left corner, right corner} {--(trifill.\i)} --cycle;
    \clip (tridashed.apex) foreach \i in {left corner, right corner} {--(tridashed.\i)} --cycle;
    \fill[black] (current bounding box.south west) rectangle (current bounding box.north east);
\end{scope}
}
\end{document}