Broken: 2.80, 2.79
Short description of error
The Extend Horizontal option of the Compositor's RGB Curves Node
does not work (acts as Extrapolated) outside the boundaries set by the Clipping option.
In the Shader Editor, Extend Horizontal works if the endpoint is in the range [0,1],
but if the endpoint is outside [0,1] it acts as the Extrapolated.