cc: @ptmerz this could be expanded

The warning is visible with clang 9.0 -Wdeprecated as:

warning: definition of implicit copy constructor for 'Invalid' is deprecated because it has a user-declared destructor

Notice that the copy constructors were removed here: fdf1a882, and this commmits
appears also in v2.5 and v2.6.

This fix should be probably backported to v2.5 and v2.6.

(cherry picked from commit 1c4eb81c and fixed for conflicts)

(should not work indeed).

See #619

might give a false positive

Should have been with 8a5eead3

(cherry picked from commit 9f411cef)

This is required to report errors with weights set to zero by mistake.

Related to #247