Is storing an invalid pointer automatically undefined behavior?

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I have the C Draft Standard here, and it makes it undefined by omission. It defines the case of ptr + I at 6.5.6/8 for

  • If the pointer operand points to an element of an array object, and the array is large enough, the result points to an element offset from the original element such that the difference of the subscripts of the resulting and original array elements equals the integer expression.
  • Moreover, if the expression P points to the last element of an array object, the expression (P)+1 points one past the last element of the array object, and if the expression Q points one past the last element of an array object, the expression (Q)-1 points to the last element of the array object.

Your case does not fit any of these. Neither is your array large enough to have -1 adjust the pointer to point to a different array element, nor does any of the result or original pointer point one-past-end.

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