CV_NODISCARD_STD was not put on the related pure virtual

Why? Users may use base class and its methods in a wrong way too.

CV_NODISCARD_STD was not put on the related pure virtual

Why? Users may use base class and its methods in a wrong way too.

Due to official c++ spec. A [[nodiscard]] attribute on a virtual is not inherited, therefore it is useless and unneeded on the pure virtual. It is only put on the two implementations.

There is no base class or implementation on a pure virtual. Therefore it is impossible to discard something which will never exist...because a pure virtual never exists.

A better way to write that is...a pure virtual never has an instance and never has an implementation. There is no definition...no code.

It is IMpossible to write myclass::mypurevirtual(x, y, z) and it work. The compiler will stop compiling with a critical error something like "no implementation".

a pure virtual defines an interface and nothing else. It is some derived class that implements it. And it is on that derived class that there can be code to execute...or have a class instance and can therefore have a nodiscard.

a pure virtual defines an interface and nothing else

Users can call this interface and they should be able get warnings.
Moreover API may not have derived classes on pubic level: just base interface + factory.

Example (Clang is smart, GCC doesn't).

Ah yes. Thanks for the use case. You're right, I'll add it. I didn't consider not creating an instance...instead passing around the pointers with vtables.

I explored and isolated that GCC fails to correctly use [[nodiscard]] on virtual functions.
If I change these to not be virtual, then GCC 7.1 correctly reports discarding. But virtual...it does not report.
gcc knows about the bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84476

MSVC works correctly.

I'll make the change and test it Thursday. I want to be sure that the python CV_WRAP doesn't have problems as the declaration would look like

CV_WRAP CV_NODISCARD_STD virtual Ptr<DescriptorMatcher> clone( bool emptyTrainData=false ) const = 0;

I want to be sure that the python CV_WRAP doesn't have problems

Perhaps we need to update this list (perhaps we need to have 1 item per line):

https://github.com/opencv/opencv/blob/4.5.3/modules/python/src2/hdr_parser.py#L437-L443

decl_str = self.batch_replace(decl_str,
    [
        ("static inline", ""),
        ("inline", ""),
        ("explicit ", ""),
        ("CV_EXPORTS_W", ""),
        ("CV_EXPORTS", ""),
        ("CV_CDECL", ""),
        ("CV_WRAP ", " "),
        ("CV_INLINE", ""),
        ("CV_DEPRECATED", ""),
        ("CV_DEPRECATED_EXTERNAL", ""),
        ("CV_NODISCARD_STD", ""),
        ("CV_NODISCARD", ""),
    ]).strip()

force pushed, includes nodiscard on pure virtual + add nodiscard handling to wrapper parser
I followed python formatting style of similar list in the same file (line 603)
python module tests passed on my local setup