/in/foo.c: In function 'calc':
/in/foo.c:26:14: warning: type defaults to 'int' in declaration of 'f_val' [-Wimplicit-int]
   26 |         auto f_val = [&](double v) {
      |              ^~~~~
/in/foo.c:26:22: error: expected expression before '[' token
   26 |         auto f_val = [&](double v) {
      |                      ^
/in/foo.c:26:36: error: expected ',' or ';' before '{' token
   26 |         auto f_val = [&](double v) {
      |                                    ^
/in/foo.c:30:16: error: called object 'f_val' is not a function or function pointer
   30 |         while (f_val(right) < 0) right *= 2.0;
      |                ^~~~~
/in/foo.c:26:14: note: declared here
   26 |         auto f_val = [&](double v) {
      |              ^~~~~
/in/foo.c:35:17: error: called object 'f_val' is not a function or function pointer
   35 |             if (f_val(mid) < 0) left = mid;
      |                 ^~~~~
/in/foo.c:26:14: note: declared here
   26 |         auto f_val = [&](double v) {
      |              ^~~~~