foo.c: In function 'main':
foo.c:6:9: warning: statement with no effect [-Wunused-value]
    6 |         for (a; a <= b; a++)
      |         ^~~
foo.c:8:32: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
    8 |                 if (a % 4 == 0 && a % 100 != 0 || a % 400 == 0)
      |                     ~~~~~~~~~~~^~~~~~~~~~~~~~~

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