• Version: v20.9.0
• Platform: Linux JABAILE-DESK02 5.15.90.1-microsoft-standard-WSL2 Detailed coverage #1 SMP Fri Jan 27 02:56:13 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux

I can only apologize in advance for this one...

I work on TypeScript, and I've been wanting to set up coverage information for the compiler. c8 does a great job; significantly faster than nyc (of course). I use c8 in my own projects with success.

However, capturing coverage for the compiler produces results that are too high to believe. checker.ts is a 50k line file, and yet its coverage is:

checker.ts | 99.99 | 95.38 | 98.62 | 99.99 | 50577-50578

There's no way that we actually have tested every line of the checker. And indeed, adding a function to a known-dead path (like one ending in Debug.assertNever) indicates that the line is covered when it definitely is not, and the overall count doesn't change.

I don't know if I have a good repro for this besides npx c8 npm test -- --no-lint on our repo. Obviously that's huge but I don't really know where to start.

At best, I can observe that npx c8 npm test -- --no-lint --no-bundle works better, i.e. not using esbuild and instead running the tsc output produces a more believable coverage count:

checker.ts  |    97.5 |    95.87 |   98.62 |    97.5 | ...-48684,48686-48687,48761-48762,48 ...