Partial \hhline causes cut in thicker-than-normal vertical line

Question

In this example, the first and second \hhlines create the farmost left border but it's really thin and doesn't match the width of the rest of the vertical line... I tried adding [4pt] after the initial | (in \hhline) but it doesn't result in the wanted behaviour. It works as expected when the line goes all the way... How can I solve this issue so that the left vertical line looks good when used together with partial \hhlines?

\documentclass{article} 
\usepackage{hhline} 
\usepackage{tabu}

\begin{document}   
    \begin{tabu}{|[4pt]ll|}
        \hhline{|--|}
            \multicolumn{2}{|[4pt]r|}{partial w multicolumn} \\
        \hhline{|~-|}    
        \hhline{|~-|}
            partial w & regular columns \\
        \hhline{|~-|}    
        \hhline{|~-|}
            non-partial w & regular columns \\
        \hhline{|--|}    
        \hhline{|--|}
            & \\
        \hhline{|--|}   
    \end{tabu} 
\end{document}

Answer 1

You can redefine \vline to make a wide rule then define itself back to normal....

\documentclass{article} 
\usepackage{hhline,colortbl} 
\usepackage{tabu}
\let\xvline\vline
\begin{document}   
    \begin{tabu}{|[4pt]ll|}
        \hhline{|--|}
            \multicolumn{2}{|[4pt]r|}{partial w multicolumn} \\
\noalign{\gdef\vline{\vrule width 4pt\global\let\vline\xvline}}
        \hhline{|~-|}    
\noalign{\gdef\vline{\vrule width 4pt\global\let\vline\xvline}}
        \hhline{|~-|}
            partial w & regular columns \\
\noalign{\gdef\vline{\vrule width 4pt\global\let\vline\xvline}}
        \hhline{|~-|}    
\noalign{\gdef\vline{\vrule width 4pt\global\let\vline\xvline}}
        \hhline{|~-|}
            non-partial w & regular columns \\
        \hhline{|--|}    
        \hhline{|--|}
            & \\
        \hhline{|--|}   
    \end{tabu} 
\end{document}