Example code:

#include <string>
struct A {
  void test(std::string s);
};

Right-clicking on "test" and selecting "Refactor" -> "Add Definition Outside Class" results in the following code:

void A::test(std::__cxx11::string s)
{
    
}

Clearly, the type should be "std::string", not "std::__cxx11::string".

Possible workaround: Disable GCCs C++11 ABI

QMAKE_CXXFLAGS += -D_GLIBCXX_USE_CXX11_ABI=0

This issue seems to be closely related to QTCREATORBUG-15461.