struct Blah {
  static const int len = 5;
};
void Foo() {
  Blah b;
  char x[Blah::len];
  char y[b.len];
}

% clang -Wvla -fsyntax-only test.cc
test.cc:7:9: warning: variable length arrays are a C99 feature, accepted as an
      extension [-Wvla]
  char y[b.len];
        ^
1 warning generated.