See https://gcc.gnu.org/pipermail/gcc-help/2024-March/143369.html for reference.

The documentation at https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html states that VLAs are supported in C++ as an extension.

Compiling the following:

void f(int a, int b[a]);
void f() {
  int c[2];
  f(2, c);
}

with g++ -std=gnu++20 results in the error: "use of parameter outside function body before ']' token".  The docs say nothing about partial support of VLA, just that they can be used.

Where this is useful as an extension is in the somewhat reasonable case of including a C header in a C++ program.  If that C header declares a function using a C99 VLA, it would be awesome if g++ were to accept it in -std=gnu++ mode (and it would be fine if it were rejected in -std=c++ mode).  Consider a situation where you cannot modify the header, and so have to use it as-is.

For comparison, clang added support for this feature in version 12.  It would be advantageous to have feature parity given that it started as a GNU extension.  Moreover, better interoperability with C is useful in its own right.