constexpr member function with std::vector data member in C++

I am trying to implement in a C++ class a constexpr member function which returns a template parameter. The code is supposed to be c++11 compatible. However I encounter compilation issues when the templated class also contains STL containers as data members such as std::vector (which are untouched by the constexpr member function).

A minimal example is given by the following code:

#include <vector>
#include <iostream>
#include <array>


template<size_t n>
struct A 
{

  constexpr size_t dimensions() const
  {
    return n;
  }
private:
  std::vector<double> a;
};


int main(int argc,char ** argv)
{
  auto a=A<3>();
  std::array<double,a.dimensions()> arr;

}

The code compiles correctly with the commands

g++ -std=c++14 -O3 quickTest.cpp -o test -Wall

clang++ -std=c++11 -O3 quickTest.cpp -o test -Wall

but fails when I use

g++ -std=c++11 -O3 quickTest.cpp -o test -Wall

with the error

quickTest.cpp:22:33: error: call to non-‘constexpr’ function ‘size_t A<n>::dimensions() const [with long unsigned int n = 3; size_t = long unsigned int]’
   std::array<double,a.dimensions()> arr;
                     ~~~~~~~~~~~~^~
quickTest.cpp:10:20: note: ‘size_t A<n>::dimensions() const [with long unsigned int n = 3; size_t = long unsigned int]’ is not usable as a ‘constexpr’ function because:
   constexpr size_t dimensions() const
                    ^~~~~~~~~~
quickTest.cpp:22:33: error: call to non-‘constexpr’ function ‘size_t A<n>::dimensions() const [with long unsigned int n = 3; size_t = long unsigned int]’
   std::array<double,a.dimensions()> arr;
                     ~~~~~~~~~~~~^~
quickTest.cpp:22:33: note: in template argument for type ‘long unsigned int’

I am using gcc 8.1.1 and clang 6.0.1