The about $1099 Voigtlander Heliar-Hyper Wide 10mm f/5.6 Aspherical for Sony E mount is the widest full frame lens on the market as far as I am aware. It is also available in M-Mount. Indeed, it is a tweaked rangefinder design.
These examples show a variety of distances to get an idea of overall imaging behavior using images at close range and distance and spanning the range to assess just how capable the lens is.
Image sizes up to 28 megapixels pixels, with crops.