What is error?

[vpi764]

void g(auto x) { g(x - 1); }

[lastchance]

Absence of declared type for the argument of the function.

Absence of any means of stopping the recursion.

Absence of any context.

[Zaap]

Probably an infinite loop.

g(2) call g(1)

g(1) call g(0)

g(0) call g(-1) etc...

You should put something like this :

if(!x) return;

[Ganado]

There is no error.

void g(auto x) { g(x - 1); }

int main()
{

}

This code gives me no error.

[lastchance]

I got

test.cpp:1:8: warning: use of 'auto' in parameter declaration only available with -fconcepts
 void g(auto x) { g(x - 1); }

Go on, stick the line
g(10);
in the middle of int main() and find out how big the stack is(n't).

[Ganado]

Well funny you should mention that, if you compile with -O3, I believe g++ does tail-call optimization, and it doesn't crash for me.

But even without a stack overflow, the program will have undefined behavior once x - 1 underflows after being the lowest int possible.