Catalina C++: Using headers yield error: no member named 'signbit' in the global namespace

Question

After upgrading to Catalina from Mojave, Setuping: /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk in the env.

I\

Answer 1

Summary: in my case, the build script was using an older version of ios-cmake toolchain (2.1.2), and updating it to 3.1.2 fixed the cmath/math include issue.

Adapting the nifty command proposed by @Ryan H. gcc -Wp,-v -E - for my case (clang, c++, iOs target)

clang -x c++ -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.4.sdk -Wp, -v -E -

yields on two Catalina including a virgin one where only tool ever installed is XCode 11.14.1:

 clang -cc1 version 11.0.3 (clang-1103.0.32.59) default target x86_64-apple-darwin19.4.0
ignoring nonexistent directory "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.4.sdk/usr/include/c++/v1"
ignoring nonexistent directory "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.4.sdk/usr/local/include"
ignoring nonexistent directory "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.4.sdk/Library/Frameworks"
#include "..." search starts here:
#include <...> search starts here:
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/11.0.3/include
 /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.4.sdk/usr/include
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include
 /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.4.sdk/System/Library/Frameworks (framework directory)
End of search list.

So the correct include path is the first non-ignored one, everything should work OK, but it didn't. It appears the issue came from an additional include command appended to the compilation call by the ios-cmake toolchain:

CompileC /Users/<...>/build.Release.ios/<...>.o <...>.cpp normal arm64 c++ com.apple.compilers.llvm.clang.1_0.compiler
-Isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.4.sdk <...>
 -I/Users/<...>/Build_iOS/build.Release.ios/build.arm/Binaries/Release/include
 -Isystem /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.4.sdk/usr/include
 -I/Users/<...>/Build_iOS/build.Release.ios/build.arm/src/<...>.build/Release-iphoneos/<...>/DerivedSources/arm64
...

The culprit was the -Isystem ... line, which will cause the #include $$ line in the cmath file to end up loading the wrong file. After much fiddling around trying to fix the cmake scripts, I noticed the older version of ios-cmake, and updating it had the 'only' effect of removing the unwanted -Isystem line - everything else was almost the same (apart from a few compiler option)