Improved Variant support

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Improved Variant support

Adam Wulkiewicz
Hi,

Currently the use of Variants (and in general run-time Geometries) can be problematic. It's because if Variant objects are passed into an algorithm the code for all possible combinations of types is generated which results in a compile-time error. Example:

within(pt, poly); // compiles
//within(poly, pt); // fails - not implemented

boost::variant<pt_t, poly_t> v1, v2;
within(v1, v2); // does not compile

The above code doesn't compile because the code calling within(poly, pt) is generated by the compiler.

I prepared a PR changing it:
https://github.com/boostorg/geometry/pull/290

With this change the above call for variants compiles and runs. Furthermore the following code compiles and throws an exception.

v1 = poly;
v2 = pt;
within(v1, v2); // compiles but throws


Does someone have something against this change?

Regards,
Adam

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