foo.c: In function 'main':
foo.c:7:13: error: 'i' undeclared (first use in this function)
    7 |     char ab[i];
      |             ^
foo.c:7:13: note: each undeclared identifier is reported only once for each function it appears in
foo.c:7:10: warning: unused variable 'ab' [-Wunused-variable]
    7 |     char ab[i];
      |          ^~

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