is not under 'rootDir' 'src/'. even with exclude

<!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨 Half of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Even if you think you've found a *bug*, please read the FAQ first, especially the Common "Bugs" That Aren't Bugs section! Please help us by doing the following steps before logging an issue: * Search: https://github.com/Microsoft/TypeScript/search?type=Issues * Read the FAQ: https://github.com/Microsoft/TypeScript/wiki/FAQ Please fill in the *entire* template below. --> `exclude` compiler option is not working at all! I'm specifying `exclude` to the folder `gen` (and even `./gen`) and still receiving the error below. <!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --> **TypeScript Version:** 3.3.3333 <!-- Search terms you tried before logging this (so others can find this issue more easily) --> **Search Terms:** **Code** ```ts // A *self-contained* demonstration of the problem follows... // Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc. ``` **Expected behavior:** No errors. **Actual behavior:** ``` error TS6059: File '.../ts-lang/gen/grammar/MyLangLexer.ts' is not under 'rootDir' 'src/'. 'rootDir' is expected to contain all source files. ``` **Playground Link:** <!-- A link to a TypeScript Playground "Share" link which demonstrates this behavior --> **Related Issues:** <!-- Did you find other bugs that looked similar? -->