I've recently switched from using Xcode 15 under Ventura to using Xcode 26 under Sequoia on an Intel-CPU Mac. This is a macOS project that uses a legacy build location, and the sources are C++ and Objective-C. For every source being compiled, I get a clang warning "no such include directory" for these 4 directories:
$BUILD_DIR/Debug/include
$BUILD_DIR/SBEngineV4.build/Debug/V4 Dual SBEngine.build/DerivedSources-normal/x86_64
$BUILD_DIR/SBEngineV4.build/Debug/V4 Dual SBEngine.build/DerivedSources/x86_64
$BUILD_DIR/SBEngineV4.build/Debug/V4 Dual SBEngine.build/DerivedSources
I can't figure out where it's getting those directories. I added a Run Script build phase that says echo $HEADER_SEARCH_PATHS, and the first path that reports is the $BUILD_DIR/Debug/include path, yet I don't see that when I look at the header search paths in the target build settings.
I tried deleting all the derived data. I tried Xcode 26.0.1 and 26.1 beta 2.