Issue with sip-6.7.8

[Jeremy Sanders]

Hi -

I'm getting a CI failure for my project with sip-6.7.8, as installed 
from the wheel on Python 3.8, on both linux and windows.

The definition in the sip file 
(https://github.com/veusz/veusz/blob/master/src/threed/threed.sip) looks 
like:

class LineSegments : public Object
{
%TypeHeaderCode
#include <objects.h>
%End
  public:
   LineSegments(const ValVector& x1, const ValVector& y1, const 
ValVector& z1,
                const ValVector& x2, const ValVector& y2, const 
ValVector& z2,
                const LineProp* prop /Transfer/);
   LineSegments(const ValVector& pts1, const ValVector& pts2,
                const LineProp* prop /Transfer/);
};

And the error when running looks like:

   File "/home/runner/work/veusz/veusz/veusz/widgets/graph3d.py", line 
169, in addBorder
     ls = threed.LineSegments(
TypeError: arguments did not match any overloaded call:
   LineSegments(x1: ValVector, y1: ValVector, z1: ValVector, x2: 
ValVector, y2: ValVector, z2: ValVector, prop: LineProp): argument 3 has 
unexpected type 'LineProp'
   LineSegments(pts1: ValVector, pts2: ValVector, prop: LineProp): 
argument 3 has unexpected type 'LineProp'
   LineSegments(a0: LineSegments): argument 1 has unexpected type 
'ValVector'

The error message doesn't make much sense, as the the second overload 
should work fine (pts1 and pts are ValVector and prop is a LineProp).

It previously built fine with sip-6.7.7.

Could this be a bug?

Thanks

Jeremy