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Thanks good catch ! just wondering how you find this one ?
Added in https://hg.libsdl.org/SDL/rev/9d6107cbcd66

Btw this was introduced in:
https://hg.libsdl.org/SDL/rev/de4288fa5b0b
( WIN_WindowProc(), case WM_ACTIVATE comparing ClipCursor )

This is a warning in clang: -Wmemsize-comparison (enabled by default).

But because SDL use SDL_memcmp, compiling with clang does not trigger the warning.

However running clang-tidy triggers the warning because SDL defines SDL_memcmp to memcmp when __clang_analyzer__ is defined: https://hg.libsdl.org/SDL/file/6f0ec1079286/include/SDL_stdinc.h#l571 .

Ok thanks for the info! 
I think I used clang-tidy some time ago.. I just re-tried, but it also gives me lots of code writing tips/warnings.

If you have a command line/script with all the interesting warning please copy paste it:)
Maybe this could be added to the Buildbots (https://buildbot.libsdl.org/waterfall) so that it's triggered on occasions.

Created attachment 4022 [details]
clang-tidy-config.sh

Created attachment 4023 [details]
modify_makefile.sh

Created attachment 4024 [details]
improve tidy chack

I attached the script I use to configure clang tidy (clang-tidy-config.sh).
It includes instructions to recreate it (for when the next version of clang, that will add new check, is out).
You can add '-' in front of a check to disable it. Then you run the script and place its output file in the root directory of the project. As long as the config file name is '.clang-tidy', it will be used automatically when you run clang-tidy.

To run clang-tidy on SDL:
mkdir ./build-clang-tidy
cd ./build-clang-tidy
./configure CC="clang-9" CFLAGS="-Wall -Wextra -Wno-sign-compare -Wno-unused-parameter -Wformat=2 "

Then I modify the Makefile with 'sed' (modify_makefile.sh):
 - set 'CC' value to 'clang-tidy'
 - modify 'all' dependencies to skip rules that create so/dll files.
 - modify object rules for clang-tidy syntax and adding 'touch $@' so that next time you run clang-tidy only modified files will be checked.

make -j4 1>./tidy.log

I also attached a patch to improve clang-tidy result:
 - defining some SDL function to the libc one, like what is done in SDL_stdinc.h
 - let clang-tidy know that SDL_SetError always return -1.

Thanks for the info and I confirm the scripts work well!

Fixed, based on clang-tidy, few readability checks, redundant things :

https://hg.libsdl.org/SDL/rev/6225152b6804
https://hg.libsdl.org/SDL/rev/3965c9d391e3
https://hg.libsdl.org/SDL/rev/6340e9291e64
https://hg.libsdl.org/SDL/rev/057a3fe53d5a
https://hg.libsdl.org/SDL/rev/c88f111dc41f
https://hg.libsdl.org/SDL/rev/1254722e1722
https://hg.libsdl.org/SDL/rev/fac2d8c66fa6
https://hg.libsdl.org/SDL/rev/15c666c579f1
https://hg.libsdl.org/SDL/rev/51d8cbeda847
https://hg.libsdl.org/SDL/rev/9d978fff9873
https://hg.libsdl.org/SDL/rev/8340a18b20bf
https://hg.libsdl.org/SDL/rev/4389d1796f6d
https://hg.libsdl.org/SDL/rev/f872396dcc58
https://hg.libsdl.org/SDL/rev/802516cf7035