Find all composite positive integers \(m\) such that, whenever the product of two positive integers \(a\) and \(b\) is \(m\), their sum is a power of $2$.

[i]Proposed by Harun Khan[/i]
To find all composite positive integers \(m\) such that, whenever the product of two positive integers \(a\) and \(b\) is \(m\), their sum is a power of 2, we will proceed with the following steps:

1. **Claim 1: \(m\) is odd.**

   *Proof:* Suppose \(m\) is even. Then \(m\) can be written as \(2^k \cdot n\) where \(n\) is odd and \(k \geq 1\). Consider the pair \((2^k, n)\). Their product is \(m\) and their sum is \(2^k + n\). Since \(n\) is odd, \(2^k + n\) is odd and greater than 1, hence it cannot be a power of 2. This contradicts the given condition. Therefore, \(m\) must be odd. \(\blacksquare\)

2. **Claim 2: \(m\) is square-free.**

   *Proof:* Suppose \(m\) is not square-free. Then there exists an odd prime \(p\) such that \(p^2\) divides \(m\). Let \(m = p^2 \cdot n\) for some integer \(n\). Consider the pair \((p, p \cdot n)\). Their product is \(m\) and their sum is \(p + p \cdot n = p(1 + n)\). Since \(p\) is an odd prime, \(p(1 + n)\) is not a power of 2, which contradicts the given condition. Therefore, \(m\) must be square-free. \(\blacksquare\)

3. **Claim 3: \(m\) has at most two distinct prime factors.**

   *Proof:* Suppose \(m\) has three or more distinct prime factors. Let \(m = p_1 p_2 \cdots p_k\) where \(k \geq 3\) and \(p_1, p_2, \ldots, p_k\) are distinct odd primes. Consider the pairs \((1, p_1 p_2 \cdots p_k)\) and \((p_1, p_2 p_3 \cdots p_k)\). Their products are \(m\) and their sums are \(1 + p_1 p_2 \cdots p_k\) and \(p_1 + p_2 p_3 \cdots p_k\), respectively. Since \(1 + p_1 p_2 \cdots p_k > p_1 + p_2 p_3 \cdots p_k\), we have:
   \[
   1 + p_1 p_2 \cdots p_k \equiv 0 \pmod{p_1 + p_2 p_3 \cdots p_k}
   \]
   This implies \(1 - p_1^2 \equiv 0 \pmod{p_1 + p_2 p_3 \cdots p_k}\), which means \(p_1^2 - 1\) is divisible by \(p_1 + p_2 p_3 \cdots p_k\). However, \(p_1 + p_2 p_3 \cdots p_k > 1 + p_1^2 > p_1^2 - 1 > 0\), which is a contradiction. Therefore, \(m\) must have at most two distinct prime factors. \(\blacksquare\)

4. **Conclusion: \(m = pq\) for some distinct odd primes \(p\) and \(q\).**

   We now solve for \(m = pq\) where \(p\) and \(q\) are distinct odd primes. We need to satisfy the conditions:
   \[
   pq + 1 = 2^a \quad \text{and} \quad p + q = 2^b
   \]
   where \(a > b\). This gives us:
   \[
   (p+1)(q+1) = 2^b (2^{a-b} + 1) \quad \text{and} \quad (p-1)(q-1) = 2^b (2^{a-b} - 1)
   \]
   For any odd integer \(k\), exactly one among \(\nu_2(k-1)\) and \(\nu_2(k+1)\) equals 1, and the other is greater than 1. We cannot have \(\nu_2(p-1) = \nu_2(q-1) = 1\) nor \(\nu_2(p+1) = \nu_2(q+1) = 1\), because both lead to \(\nu_2((p-1)(q-1)) \neq \nu_2((p+1)(q+1))\), which is not the case since both are equal to \(b\).

   Therefore, we assume without loss of generality that \(\nu_2(p-1) = \nu_2(q+1) = 1\). This forces \(\nu_2(p+1) = \nu_2(q-1) = b-1\), leading to:
   \[
   p + q = (p+1) + (q-1) \geq 2^{b-1} + 2^{b-1} = 2^b
   \]
   Since equality occurs, we have \(p+1 = q-1 = 2^{b-1}\). Considering this modulo 3, one of \(p\) or \(q\) must be divisible by 3. Therefore, \(p = 3\) and \(q = 5\), which gives \(m = 3 \cdot 5 = 15\).

The final answer is \(\boxed{15}\).