Experiencing the same on macOS 15.0.1 with Xcode 16.0. The iOS target does not compile, giving errors related to ___wctype.h: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/include/___wctype.h:83:24 Use of undeclared identifier '_CTYPE_A' /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/include/___wctype.h:83:33 Use of undeclared identifier '_CTYPE_D' ... /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/include/___wctype.h:149:25 Use of undeclared identifier '_CTYPE_X' Never seen this issue before. Only the iOS target does not compile in the project (regardless whether minimum deployment set to iOS 15 - iOS18). Build settings Base SDK has "iOS" set (without version numbers). The 2 other macOS targets compile in the project. If I create a new empty iOS target, that also compiles, however I can't compile the existing iOS target.

Based on one of the answers above "Xcode 16 appears to have changed the header search behavior." I asked chatGPT: Yes, Xcode 16 has indeed introduced changes in how it handles header search paths, and this might be the root cause of the issues you’re seeing with undeclared identifiers such as _CTYPE_A in headers like ___wctype.h. These changes can affect projects that depend on third-party libraries or custom SDK setups, especially if they modify search paths or include files in a non-standard way. Go to your project’s Build Settings. Look for Header Search Paths and ensure they are set up correctly. • You may need to change from recursive to non-recursive to prevent unintended overrides. • Consider moving system paths like /usr/include or /Applications/Xcode.app/Contents/Developer above any custom or third-party paths. • Try adjusting the order of the search paths to ensure that system headers take precedence over any third-party or user-defined paths. I was able to resolve this under Build Settings by changing a custom path I have set for Header Search Paths, from recursive to non-recursive.