When LLVM-Clang version 9 is used to compile Qt Creator there is a problem with ClangFormat plugin: line 63 of /src/plugins/clangformat/clangformatutils.cpp:

style.AllowShortIfStatementsOnASingleLine = false;

produces "uncompatible type assignation" error.

 
Now the correct workaround is to replace that line to the following:

style.AllowShortIfStatementsOnASingleLine = clang::format::FormatStyle::SIS_Always;