These options control whether an implicitly instantiated template member function can be selected as a key function. (Normally the key, or decider, function for a class is its first non-inline virtual function, in declaration order, that is not pure virtual. However, in the case of an implicitly instantiated template function, the function would have vague linkage, that is, might be multiply defined.)

Remark #2819-D is produced when a key function is implicit. This remark can be seen with --remarks or with --diag_warning=2819.