Consider the following c++ program:

  class A
  {
      protected:
          int x;
  };

  template<typename X>
  using B = A;

  template<typename T>
  class C : public B<T>
  {
      public:
          void f()
          {
              x = 0;
          }
  };

  int main()
  {
  }

Compile it with "-std=c++17 -pedantic-errors".

Expected behaviour:
  
  Since the base class is not dependent (see http://wg21.link/cwg1390 ) the
  name lookup of x should succeed and no error message should be outputed
  during the compilation.

Observed behaviour:
  
  An compilation error about failing the name lookup of x was outputed during
  the compilation.

GCC is correctly compiling the program with no error messages outputed. See the discussion in: https://bugs.llvm.org/show_bug.cgi?id=47435