So here's a fun thought experiment: What if Tesla Inc. (TSLA) and SpaceX merged into one mega-Elon-Musk-company? According to investor Gary Black, the answer is: "A solution looking for a problem." And not just any problem—a 25% haircut on Tesla's stock value.

Black, managing director of The Future Fund LLC, took to X on Sunday to warn Tesla bulls about the potential consequences of combining Elon Musk's electric vehicle and space exploration empires. His message was clear: This would be bad news for Tesla shareholders.

"If TSLA with a 100x EV/EBITDA and $1.5T market cap buys SpaceX with a 200x EV/EBITDA and $1.5T market cap," Black wrote, you'd see a "20-25% reduction" in Tesla's stock value.

Let's break down his math, because this is where it gets interesting. Black's analysis goes like this: Tesla issues $1.5 trillion in new equity to buy $1.5 trillion worth of SpaceX equity. The combined entity now has $3 trillion in equity and $22.5 billion in EBITDA (that's earnings before interest, taxes, depreciation, and amortization, for those keeping score at home).

Here's the breakdown: Tesla brings $15 billion in annual EBITDA to the party, while SpaceX contributes $7.5 billion. But here's the catch—after the merger, the combined company would likely trade at Tesla's lower valuation multiple of 100x EV/EBITDA, not SpaceX's higher 200x multiple.

"Post merger TSLA/SpaceX should trade at a 100x EV/EBITDA so $2.25T," Black explained. That $2.25 trillion valuation represents a 25% reduction from the pre-merger combined value of $3 trillion. Black calls this the "conglomerate discount"—the market's tendency to value diversified companies less than the sum of their parts.

"In my 30 years as a professional investor I have rarely seen post-merger companies trade at blended multiples based on the underlying companies' respective multiples and growth prospects," Black said. Translation: When you mix a high-flying space company with an electric vehicle maker, you don't get the average of their valuations. You get the lower one.

Now, what about the reverse scenario? What if SpaceX bought Tesla instead? Black acknowledges this "could result in short-term gains for TSLA shareholders," but warns it might come with unintended consequences. Early Tesla investors who bought into the company for its "EVs, autonomy, and robots" vision might decide to cash out if their stock suddenly becomes part of a space company.

"With conglomerates, the least common multiple generally prevails," Black noted, adding that Berkshire Hathaway Inc. (BRK) is "the only exception to the rule." Warren Buffett's conglomerate somehow manages to avoid the discount that plagues most diversified companies, but Black suggests Tesla-SpaceX wouldn't be so lucky.

"It's dilutive for $TSLA shareholders and so unlikely to happen," Black concluded, putting a pretty definitive period on the discussion.

This warning comes at an interesting time for both companies. Musk remains bullish on Tesla's Terafab goal—a massive chip fabrication project that's crucial for the company's self-driving ambitions. Despite skeptics warning about the scale and risk, Musk is pushing forward with what would essentially be two separate chip fabs.

Meanwhile, SpaceX is reportedly gearing up for an IPO that could happen as soon as June. Musk has been talking up both companies' AI capabilities, suggesting his enterprises would exceed their rivals in AI development and positioning SpaceX as "the leader in space-based AI."

So while the merger talk might be speculative, the underlying dynamics are very real. Tesla investors are betting on electric vehicles, autonomy, and robots. SpaceX investors (or future IPO buyers) are betting on space exploration and satellite networks. Mixing those stories together creates what Black calls a "solution looking for a problem"—and potentially a 25% problem for Tesla shareholders at that.

As for the stock itself, Tesla slid 3.24% to $367.96 at market close on Friday, but gained 1.17% to $372.25 during the after-hours session. Whether Black's warning about a potential SpaceX merger factors into future price movements remains to be seen, but it certainly gives investors something to think about beyond the usual quarterly earnings and delivery numbers.