foo.cc: In function 'int check(int)':
foo.cc:9:22: error: 'sqrt' was not declared in this scope
     for (i = 2; i <= sqrt(x); i++)
                      ^~~~
foo.cc:9:22: note: suggested alternative: 'qsort'
     for (i = 2; i <= sqrt(x); i++)
                      ^~~~
                      qsort

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