That’s right - sorry I never mentioned - just took it for granted…
Yes, there is a quick method - in object select mode, go to surfaces tab, select Coons surface and select the line - boom - done. Pretty ugly and not good if you are going to work any further on it. Item 1 in the pic.
A better way is to select the line, hold Ctrl and drag a bit downwards on the little yellow square in the centre of the universal manipulator - item 2.
Straighten the top and bottom lines by dragging on the green square on the Y-axis of the manipulator - be careful not to override the “corner” edges if your shape is like mine - item 3.
Bridge the top and bottom edges - it helps if the original semi-circles have an equal number of points, but that’s not absolutely necessary - just makes it neater and easier. You will need to cut in extra edges so as to make the “corner” polys into quads. - item 4.
Suggestion - it helps to attach screen shots when you ask for assistance - a picture is worth a thousand words:-)
Click thumbnail to see full-size image