There are a few bugs in Swift 4.2.1 (2017) with Float80:

example: Float80(2.718281828459045312) truncates to Float64 then stores the inaccurate result in Float80 give the result: $R75: Float80 = 2.7182818284590450908.

Likewise let q:Float80 = Float80(2.718281828459045312) gives the result:q: Float80 = 2.7182818284590450908

The only way to get by that bug is by giving up on the Float80() method altogether: let q:Float80 = 2.718281828459045312 correctly gives the result: q: Float80 = 2.71828182845904531197