Babili's sourcemaps causes error in collapseSourcemaps.js

[Comandeer]

I've created simple plugin for babili, however sourcemaps don't work.

Stack trace

TypeError: Cannot read property 'traceSegment' of undefined
      at node_modules/rollup/dist/rollup.js:8902:23
      at Array.forEach (native)
      at node_modules/rollup/dist/rollup.js:8900:8
      at Array.map (native)
      at Link.traceMappings (node_modules/rollup/dist/rollup.js:8897:31)
      at collapseSourcemaps (node_modules/rollup/dist/rollup.js:9026:19)
      at Bundle$$1.render (node_modules/rollup/dist/rollup.js:9611:10)
      at Object.generate (node_modules/rollup/dist/rollup.js:9800:26)
      at rollup.then (tests/index.js:97:26)

It can be simply reproduced by running tests from my plugin, especially ones dedicated to sourcemaps.

After some investigation I discovered that Babili often returns last line mapping with only one segment:

Example Babili's output

[ [ [ 0, 0, 0, 0, 0 ],
    [ 8, 0, 0, 8, 1 ],
    [ 11, 0, 0, 0 ],
    [ 12, 0, 0, 13 ],
    [ 16, 0, 0, 0 ],
    [ 17 ] ] ]

This causes that source variable inside Link.traceMappings is undefined, so all efforts to use that variable (especially the very next line of code) just fails.

The simplest fix would be probably to just check if source variable is really set before trying to use it.

I also know that this issue seems to be very similar to #961 but my very quick research with GCC shows that the fix does not apply there and GCC still produces invalid sourcemaps.

[Rich-Harris]

Released 0.41.5 with the fix — thanks, and sorry for the delay in merging