namespace xyz
{
  namespace abc {  }

  class MyClass
  {
  public:
    MyClass(void);
    void fooFunc(int a); // Alt-G on fooFunc does nothing
    void fooFunc(bool b); // Alt-G on fooFunc jumps to above fooFunc()
    virtual ~MyClass(void);
  };
}

#include "myclass.h"

using namespace xyz;
using namespace abc; // <<<---- source of the problem

MyClass::MyClass(void)
{}

MyClass::~MyClass(void)
{}

void MyClass::fooFunc( int a ){} // Alt-G on fooFunc offers the both 
                                 // fooFunc() from the myclass.h

void MyClass::fooFunc( bool b ){}// Alt-G on fooFunc offers the both 
                                 // fooFunc() from the myclass.h

namespace xyz
{
  namespace abc {  }

  class MyClass
  {
  public:
    MyClass(void);
    void fooFunc(int a); // Alt-G on fooFunc does nothing
  };
}

#include "myclass.h"

using namespace xyz;
using namespace xyz::abc; // if you remove this Alt-G will work again

MyClass::MyClass(void) { }

void xyz::MyClass::fooFunc( int a ) { } // Alt-G jumps to fooFunc in myclass.h
  // ^^^^^--- alternatively if you remove this Alt-G will work again