Type guard does not narrow the same as if-statement

<!-- 🚨 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. --> <!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --> **TypeScript Version:** 3.5.1 <!-- Search terms you tried before logging this (so others can find this issue more easily) --> **Search Terms:** + Type guard function + If-statement guard + Narrow + Discriminated union **Code** ```ts enum E1 { element0 = "element0", element1 = "element1", element2 = "element2", } enum E2 { element3 = "element3", element4 = "element4", element5 = "element5", } declare const obj : ( | { type : E1, data : { object : { x : "e1" } } } | { type : E2, data : { object : { x : "e2" } } } ); if (obj.type == E1.element0) { //OK const type : "element0" = obj.type; //OK const x : "e1" = obj.data.object.x; } if ( obj.type == E1.element0 || obj.type == E1.element1 || obj.type == E1.element2 ) { //OK const type : "element0"|"element1"|"element2" = obj.type; //OK const x : "e1" = obj.data.object.x; } declare function isE1 (mixed : unknown) : mixed is E1; if (isE1(obj.type)) { //OK const type : "element0"|"element1"|"element2" = obj.type; //Expected : OK //Actual : Error; Type '"e1" | "e2"' is not assignable to type '"e1"'. const x : "e1" = obj.data.object.x; } ``` **Expected behavior:** 2nd and 3rd examples sho…