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#include <stdio.h>
 
#define I_HATE_MACRO2(a, b) a ## b
#define I_HATE_MACRO(a, b) I_HATE_MACRO2(a, b)
#define ARRAY_LENGTH(len, a) \
    typedef struct { \
        int I_HATE_MACRO(must_be_an_array_,__LINE__) : ((void*)&(a) == (void*)(a)); \
    } I_HATE_MACRO(must_be_an_array_,__LINE__); \
    const size_t len = sizeof(a) / sizeof(a[0]);
 
int main() {
    int test[] = {1,2,3};
    int test2;
    int *test3;
 
    ARRAY_LENGTH(testlen, test);
    ARRAY_LENGTH(testlen2, test2);
    ARRAY_LENGTH(testlen3, test3);
 
    printf("%d\n", testlen);
 
    return 0;
}

prog.c: In function ‘main’:
prog.c:17: error: bit-field ‘must_be_an_array_17’ width not an integer constant
prog.c:17: error: subscripted value is neither array nor pointer
prog.c:18: error: bit-field ‘must_be_an_array_18’ width not an integer constant
prog.c:18: warning: unused variable ‘testlen3’
prog.c:17: warning: unused variable ‘testlen2’

Standard output is empty