<!-- BUGS: Please use this template. --> <!-- QUESTIONS: This is not a general support forum! Ask Qs at http://stackoverflow.com/questions/tagged/typescript --> <!-- SUGGESTIONS: See https://github.com/Microsoft/TypeScript-wiki/blob/master/Writing-Good-Design-Proposals.md --> <!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --> **TypeScript Version:** 2.7.0-dev.201xxxxx **Code** ```ts const foo: [number, string][] = [[1, 'one']]; console.log(foo.concat([2, 'two'])); ``` **Expected behavior:** Either `[[1, 'one'], [2, 'two']]` or a type error. **Actual behavior:** `[[1, 'one'], 2, 'two']`