Merge pull request #15168 from roberth/fix-protocol-addToStore-version-comparisons

Add -Werror=c99-designator and fix brace elision warnings

nix-meson-build-support/common/meson.build

@@ -40,6 +40,11 @@ do_pch = cxx.get_id() == 'clang'
 # instantiations in libutil and libstore.
 if cxx.get_id() == 'clang'
   add_project_arguments('-fpch-instantiate-templates', language : 'cpp')
+  # Catch brace elision bugs: when WorkerProto::Version changed from `unsigned int`
+  # to `struct { unsigned int major; uint8_t minor; }`, `.version = 16` silently
+  # became `.version = {16, 0}` instead of failing, breaking protocol compatibility
+  # in a subtle way
+  add_project_arguments('-Werror=c99-designator', language : 'cpp')
 endif
 
 # Detect if we're using libstdc++ (GCC's standard library)