use getCanonicalFileName instead

👍

Rolled back using getCanonicalFileName - the change in output this caused was a nightmare for our projects tests. On first attempt, the canonical path becomes absolute in many cases - this was a big change which would pretty much require reworking the test harness' path handling, so then I tried patching the function to make the result relative if need be - this was still a change in output, because the resulting relative paths could be different than the ones found by looking for common path components (usually they'd end up with more common components). This caused absolute paths to be generated by other algorithms elsewhere which then got emitted in tests and, generally, it was a mess.

We should really swap to using fully qualified Path types for everything and convert to root-relative paths for test output (in the harness).

And now I'm using getCanonicalFileName again. 👍

Worked around the casing handling in the harness/emitter by using getCanonicalFileName on the path fragments when comparing fragments rather than on the whole initial path. This is similar to what we do within a few of our utility functions, so I suppose this use of getCanonicalFileName is to be expected, though I didn't expect it to be handled at first.

I'd extract this to a function named something like prefixRootDir(name, root) or maybe prefixRootDirIfNeeded.

isRootedDiskPath ?

Aye, that works.

Scratch that, no - we're saying > 1 because we want to exclude paths which start with /, which isRootedDiskPath would include.

Since if the root is a filesystem rooted at /, we can just use / as the common dir.

but apparently we cannot do this since commonSourceDirectory is "". Just trying to understand in what cases using isRootedDiskPath will yield incorrect results.

Correct - in effect, we want to allow "" as a valid common source directory provided either all paths are relative or all paths are rooted to a unix-style drive (with a root of /), otherwise "" is the return value when there is no available common path and we should error in its presence.

discussed offline