• should bg::distance be able to return a distance when one-point linestrings are passed to it?
• should bg::distance be able to return a distance when one of the two input geometries is a closed polygon with less than four points?

In both cases above the geometries are invalid (in the OGC sense), and this actually brings up a more general question. To what extend should we support invalid geometries in BG algorithms?

In the current version of bg::distance if the uses passes an one-point linestring the algorithm sometimes returns something meaningful, and other times an assertion is triggered. Such a behavior is IMHO in some sense okay: BG algorithms are not guaranteed to work on invalid input (but they should work with valid input). So either returning something meaningful or triggering an assertion, or even returning something not meaningful is okay in the sense that the algorithm's behavior is undefined.

Motivated by the above I decided to implement a new algorithm called is_below_minimum_size. It takes a geometry as input and returns true if the geometry's size is below the minimum acceptable valid size (see also the corresponding PR: https://github.com/boostorg/geometry/pull/193). In the PR there is a related new exception, and my intention was to use that exception instead of the empty_geometry_exception currently used in the bg::distance code. Using the new exception would avoid some assertion failures, and would treat geometries with very few points in a unified manner (through exceptions).
On the other hand, it would limit the support for bg::distance on invalid geometries.

I would like your thoughts/suggestions/comments on any of the statements made above.

All the best,

- m.