The workaround in vala-0.56.14-2.fc40 gets this package to build with GCC 14—if the C sources are regenerated during the build. I had to add this to %prep to make that happen:

diff --git a/caribou.spec b/caribou.spec
index 55d41f9..7544471 100644
--- a/caribou.spec
+++ b/caribou.spec
@@ -99,6 +99,7 @@ non-gnome-shell sessions.
 %prep
 %autosetup -p1
 autoreconf -fiv
+find -name '*.vala' -exec touch {} \;
 
 %build
 %configure --disable-static PYTHON=python3

With this change, lowering the C type safety level is not strictly required anymore.

This bug appears to have been reported against 'rawhide' during the Fedora Linux 40 development cycle.
Changing version to 40.

Thanks your solution worked.