Arguably, pointing to t/2 (in the second case) is the correct behaviour, because (if concepts are being used), then it is the function (double_it) that is mis-specifying it’s requirements. Even better would be to point at the function definition as well and say that there is nothing in the concept that allows this function.
I think the core problem (don’t know if this is still the case, haven’t actually used concepts) is that templates raise errors at the point of expansion, rather than at the point of definition, because of how they are specified/implemented.
I think it’s possible that the compiler could do something smart by auto-creating a temp type that is the minimal possible implementation of the concept and attempting to compile the function. Any errors that result, should be flagged at the concept specified in the function.
I think the core problem (don’t know if this is still the case, haven’t actually used concepts) is that templates raise errors at the point of expansion, rather than at the point of definition, because of how they are specified/implemented.
I think it’s possible that the compiler could do something smart by auto-creating a temp type that is the minimal possible implementation of the concept and attempting to compile the function. Any errors that result, should be flagged at the concept specified in the function.