Demo of Typescript module augmentation bug

Repro

1. Run yarn tsc --watch
2. Change fn on line 4 of classnames.d.ts to return a number instead of a string

Expected: Type error on line 4 of index.ts: Type 'number' is not assignable to type 'string'. Actual: No type error in CLI, though error does show in VSCode

Updates work as expected when using module declaration instead of module augmentation

1. Comment out the import statement of classnames.d.ts, turning it into a module declaration instead of a module augmentation
2. Run yarn tsc --watch. An error shows up on line 1 of index.ts, saying Module '"classnames"' has no exported member 'Argument' (this is expected since the module declaration overrode all other exports from classnames)
3. Change fn on line 4 of classnames.d.ts to return a number instead of a string

Result: Type error on line 4 of index.ts: Type 'number' is not assignable to type 'string'.