in the context of another setting I was asked the following question.
Suppose the IND-CPA game is played with a symmetric encryption function $E$ that keeps a state of how many encryptions have already been requested by the adversary $A$ during the execution of the game. $E$ always outputs $E(m)=E'(m)$, where $E'$ is an IND-CPA secure encryption scheme, except for the $i$-th request, where $E(m)$ is insecure and trivially reveals the plaintext $m$ instead of invoking $E'$. $i$ is polynomially bounded in the security parameter and $A$ does not know how $i$ is determined (it could be probabilistic and depending on the previous executions of $E$).
Intuitively, I'd say that $E$ is not IND-CPA secure. An Adversary $A$ that just guesses the polynomially bounded $i$ and poses any challenge after $i-1$ encryption requests during the pre-challenge phase would always win the game if he correctly guessed $i$, which happens with non-negligible probability as $i$ is polynomially bounded. However, when trying to do a formal proof of this, I struggle to state an explicit $A$ with a non-negligible advantage, as this $A$ needs a precise specification of the upper-bound of $i$.
I'd be thankful for further discussion and tips about how one could solve my issue.