Source: https://aur.archlinux.org/cgit/aur.git/commit/?h=freefilesync&id=7e1524db8e0243ff9d7b7dcba64a3a5228295725
Author: Simon Brulhart
Message: libssh2 v1.8.0 does not yet implement LIBSSH2_SFTP_DEFAULT_MODE, revert to previous impl.
Method:

    sed -e 's/LIBSSH2_SFTP_DEFAULT_MODE/LIBSSH2_SFTP_S_IRWXU | LIBSSH2_SFTP_S_IRWXG | LIBSSH2_SFTP_S_IRWXO/g' -i 'FreeFileSync/Source/afs/sftp.cpp'

Source: <none>
Author: B Stack
Message: libssh2 v1.8.0 does not yet implement these two error messages. I do not know why distros are not serving libssh2 v1.9.0 which wa released June 20, 2019.
Date: 2019-09-17T13:41:23Z
--- 10.17-0/FreeFileSync/Source/afs/libssh2/libssh2_wrap.h	2019-10-17 15:59:11.681641905 -0400
+++ 10.17-1/FreeFileSync/Source/afs/libssh2/libssh2_wrap.h	2019-10-21 08:32:59.666149954 -0400
@@ -172,8 +172,8 @@
             ZEN_CHECK_CASE_FOR_CONSTANT(LIBSSH2_ERROR_ENCRYPT);
             ZEN_CHECK_CASE_FOR_CONSTANT(LIBSSH2_ERROR_BAD_SOCKET);
             ZEN_CHECK_CASE_FOR_CONSTANT(LIBSSH2_ERROR_KNOWN_HOSTS);
-            ZEN_CHECK_CASE_FOR_CONSTANT(LIBSSH2_ERROR_CHANNEL_WINDOW_FULL);
-            ZEN_CHECK_CASE_FOR_CONSTANT(LIBSSH2_ERROR_KEYFILE_AUTH_FAILED);
+            //ZEN_CHECK_CASE_FOR_CONSTANT(LIBSSH2_ERROR_CHANNEL_WINDOW_FULL);
+            //ZEN_CHECK_CASE_FOR_CONSTANT(LIBSSH2_ERROR_KEYFILE_AUTH_FAILED);
     }
     return replaceCpy<std::wstring>(L"SSH status %x.", L"%x", numberTo<std::wstring>(sc));
 }
Message: It appears upstream moved libssh2_wrap, but the above file patches correctly, so we will leave it while adding this new, identical patch.
diff -Naur /usr/src/freefilesync/10.20-0/libssh2/libssh2_wrap.h /usr/src/freefilesync/10.20-1/libssh2/libssh2_wrap.h
--- 10.20-0/libssh2/libssh2_wrap.h	2020-02-15 14:50:08.356617565 -0500
+++ 10.20-1/libssh2/libssh2_wrap.h	2020-02-15 15:11:25.992842154 -0500
@@ -172,8 +172,8 @@
             ZEN_CHECK_CASE_FOR_CONSTANT(LIBSSH2_ERROR_ENCRYPT);
             ZEN_CHECK_CASE_FOR_CONSTANT(LIBSSH2_ERROR_BAD_SOCKET);
             ZEN_CHECK_CASE_FOR_CONSTANT(LIBSSH2_ERROR_KNOWN_HOSTS);
-            ZEN_CHECK_CASE_FOR_CONSTANT(LIBSSH2_ERROR_CHANNEL_WINDOW_FULL);
-            ZEN_CHECK_CASE_FOR_CONSTANT(LIBSSH2_ERROR_KEYFILE_AUTH_FAILED);
+            //ZEN_CHECK_CASE_FOR_CONSTANT(LIBSSH2_ERROR_CHANNEL_WINDOW_FULL);
+            //ZEN_CHECK_CASE_FOR_CONSTANT(LIBSSH2_ERROR_KEYFILE_AUTH_FAILED);
     }
     return replaceCpy<std::wstring>(L"SSH status %x.", L"%x", numberTo<std::wstring>(sc));
 }