Multiple implicit declaration of same function in C

Here's my code:

int main(){
    printf("Hi");
    int i=10;
    printf("Hi %d",i);
    return 0;
}

Now as C can implicitly declare a function this program will compile correctly (As it does with gcc).
But my question is, wasn't the 1st printf declared to return an int with 1 parameter of type char * ?
That makes the 2nd printf an error.
Yet the program compiles with no errors, just warnings (gcc). Why ?

Solution

Strictly speaking, implicit declaration is standard violation. It is removed from the standard.

Quoting C11, Foreword

Major changes in the second edition included:

— remove implicit function declaration

That said, in earlier version of C, for a function which is considered declared implicitly (i.e., used before the compiler has knowledge about the function prototype) was supposed to

return an int
accepts any number and type of parameter.

So, as long as the function declaration and the definition does not collide (say, return type mismatch), you'll not get any error. However, a strictly conforming compiler MUST produce a diagnostic.