I think I'd put the difference between "invariant" and "assumption" as purely a difference in framing. Both describe a problem in communication between two com

I think I'd put the difference between "invariant" and "assumption" as purely a difference in framing. Both describe a problem in communication between two components of a system. Suppose there are two components, where the output of component A is sent to component B. Both components seem to be working correctly, but when run together they are not producing the expected output. * Option 1: Component A is violating an invariant of its output type. Whether that is invalid data (e.g. nullptr), internal inconsistency (e.g. an index that will be out of bounds for its array), or some (e.g. matrices with determinant one), the output generated by Component A is wrong in some way. * Option 2: Component B is making an unjustified assumption about its input type. The implementation was only valid when the assumption held, but Component B's interface implied that it could accept a wider range of inputs. The only difference between these two cases is the human-level description of what the type is "supposed" to hold. By describing it as an "invariant", the blame is placed at Component A. By describing it as an "assumption", the blame is placed at Component B.