[Joker_vD]

> more machinery is necessary to track those dependencies.

Well, is it much more machinery? IIRC doing

    void f(size_t n) {
        int x[n];
        n += 1;
        ...

does not resize x, so there is no dataflow dependency or rather, x depends on a hidden const variable so no additional dataflow analysis necessary.

[LegionMammal978]

Yet

  void f(size_t n, int cond) {
      if (cond) { n += 1; }
      int x[n];
      ...

does resize x depending on the value of cond, so the size can't necessarily be known until the point where the type (int[n] in this case) is named.

Also, the compiler has to make sure it keeps around implicit locals to store the variable layouts, so that code like

  void f(size_t n) {
      typedef int array[n];
      n += 1;
      array x;
      ...

functions as specified. This kind of pattern is one of the bigger things setting the feature apart from just "syntax sugar for alloca()".