As discussed in bug 88766 comment #4, GCC treats some statement expressions as lvalues even though their last statement is an rvalue.  As noted in the rest of the discussion a statement expression should evaluate to an rvalue even if it's last statement is an lvalue.

Besides adjusting GCC to reflect that the manual should be updated to explain this.

$ cat u.c && gcc -O2 -S -Wall -Wextra -Wpedantic -fdump-tree-optimized=/dev/stdout u.c
void f (void)
{
  int i = 0;
  ++__extension__ ({ (int)i; });   // accepted (bug)
  if (i != 1)                      // folded to false
    __builtin_abort ();
}

int* g (void)
{
  int i = 0;
  int *p = &++__extension__ ({ (int)i; });   // rejected (good)
  return p;
}

u.c: In function ‘g’:
u.c:12:12: error: lvalue required as unary ‘&’ operand
   12 |   int *p = &++__extension__ ({ (int)i; });   // rejected (good)
      |            ^