This is the example in [expr.prim.lambda.capture]p9:

void f2() {
  int i = 1;
  void g1(int = ([i]{ return i; })()); // ill-formed
  void g2(int = ([i]{ return 0; })()); // ill-formed
  void g3(int = ([=]{ return i; })()); // ill-formed
  void g4(int = ([=]{ return 0; })()); // OK
  void g5(int = ([]{ return sizeof i; })()); // OK
}

My question is, what is the smallest enclosing scope of the lambdas in this example?

According to my understanding, it is the function prototype scope. But then the declaration of g4 is not OK: the smallest enclosing scope of the lambda is not a block scope, and so according to [expr.prim.lambda.capture]p3 no capture-default is allowed.

Note that CWG 1632 is also about [expr.prim.lambda.capture]p3 (previously [expr.prim.lambda]p9).