Having Clang version 10.0.0 will trigger trigger the following error when compiling pyside from source:

Unable to locate Clang's built-in include directory (neither by checking the environment variables LLVM_INSTALL_DIR, CLANG_INSTALL_DIR nor running llvm-config). This may lead to parse errors.

The error appear because in findClangBuiltInIncludesDir method from sources/shiboken2/ApiExtractor/clangparser/compilersupport.cpp file, the major version is considered as 1 digit and the subsequent test fails

const QVersionNumber versionNumber = QVersionNumber::fromString(fileName.at(0));
 if (!versionNumber.isNull() && versionNumber > lastVersionNumber) {

Changing

QVersionNumber lastVersionNumber(1, 0, 0);

to

QVersionNumber lastVersionNumber(0, 0, 0);

will allow compilation to continue.