To improve the error messages.

A friendly type predicate

by Andrzej Krzemieński

From the article:

This is a sequel to the previous post on writing a custom type predicate. One of the readers on Reddit made a very insightful observation. The user has implemented a type that she intends to use with our library, call it Calc. She passes it to the library function, and she gets the compiler error:

static assertion failed: X does not have a desired interface

But what is our type missing? In the previous post we were describing 3 constraints. A concept could have far more of them. The user has already made an effort to have Calc comply with the constraints, so there must be something tiny missing. Maybe there is a bug in the implementation of the predicate? But it is difficult to track what it is that the predicate does not like about our type. We could use some more specific information...