You're right that it's pretty easy to write the individual tests. But on the whole, I disagree: in my experience, compilers for non-trivial languages have an e

You're right that it's pretty easy to write the individual tests. But on the whole, I disagree: in my experience, compilers for non-trivial languages have an extremely large surface area, especially when optimization is involved. Language features have the habit of interacting with each other in non-trivial ways, and the combinatorics can quickly get overwhelming.