Make the type guarding on 'typeof' be transitive.

<!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨 --> <!-- Half of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Please help us by doing the following steps before logging an issue: * Search: https://github.com/Microsoft/TypeScript/search?type=Issues * Read the CONTRIBUTING guidelines: https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md * Read the FAQ: https://github.com/Microsoft/TypeScript/wiki/FAQ --> <!-- If you have a QUESTION: THIS IS NOT A FORUM FOR QUESTIONS. Ask questions at http://stackoverflow.com/questions/tagged/typescript or https://gitter.im/Microsoft/TypeScript --> <!-- If you have a SUGGESTION: Most suggestion reports are duplicates, please search extra hard before logging a new suggestion. See https://github.com/Microsoft/TypeScript-wiki/blob/master/Writing-Good-Design-Proposals.md --> <!-- If you have a BUG: Please fill in the *entire* template below. --> <!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --> **TypeScript Version:** 2.7.0 <!-- Search terms you tried before logging this (so others can find this issue more easily) --> **Search Terms:** typeof + transitive + type guard + typeof same **Code** ```ts function test(a: string | number, b: string | number) { if (typeof a === typeof b && typeof a === 'string') return b.length } ``` **Expected behavior:** variable `b` be asserted as string type. **Actual behavior:** TS report: `Property 'length' does not exist on type 'string | number'.` at `b.length` **Playground Link:** <!-- A link to a TypeScript Playground "Share" link which demonstrates this behavior --> [Playground](http://www.typescriptlang.org/play/index.html#src=function%20test(a%3A%20string%20%7C%20number%2C%20b%3A%20string%20%7C%20number)%20%7B%0A%20%20if%20(typeof%20a%20%3D%3D%3D%20typeof%20b%20%26%26%20typeof%20a%20%3D%3D%3D%20'string')%20return%20b.length%0A%7D) **Related Issues:**