tsc --build mode: tsc crashes in situation involving global module augmentations

[yortus]

TypeScript Version: 3.0.0-dev.20180630

Search Terms:
build mode, crash

Steps to Reproduce:

Set up @RyanCavanaugh's learn-a sample repo as per his instructions. Now run lerna add @types/multer to add multer typings to all three packages. Run tsc -b packages --forceto confirm it still builds fine. Now add the following line to pkg1/src/index.ts:

export {Options} from 'multer';

Run tsc -b packages --force again.

Expected behavior:

Builds successfully.

Actual behavior:

tsc crashes due to a violated assertion. I looked briefly at the stack trace and assertion, and it seems to be something to do with the global augmentation of the Express namespace.

Related Issues:

Originally reported in #3469 (comment).

[RyanCavanaugh]

@sandersn super interesting - this hits the target !== source assert added to mergeSymbolTable despite not having any .js files. Doesn't seem to be related to --build (can repro this under normal tsc -p once upstreams are built)