Fix hash comparison for isSameRoute (fix #635)

[fnlctrl]

Fix hash comparison for isSameRoute (#635)
when one of a.hash and b.hash is undefined while the other is empty string

This bug causes active class not being applied when using named view + exact matching

[posva]

Feels a bit hacky to be done here. Can't route get normalised somewhere else?

[fnlctrl]

Hmm.. Yes it's hacky.. I'll look deeper and see if I can normalize it elsewhere.

[fnlctrl]

By definition, isSameRoute's parameter a and b is type Route, whose hash is a required string field, but sometimes b.hash is undefined. So there may be some bug flow wasn't able to discover.

[posva]

Actually, it's is required so the check has to be done somewhere

[fnlctrl]

@posva I think I've found the cause: It's in router-link's render function, where it passed a Location object (whose hash is optional) directly into isSameRoute:
see https://github.com/vuejs/vue-router/blob/next/src/components/link.js#L31-L33

I'm surprised that flow didn't warn on implicit type conversion: passing a Location object to isSameRoute, while it asked for a Route

[fnlctrl]

To sum up, @yyx990803 A more elegant fix for this (instead of the hash || '' fix) involves modifying Location type, or a converter from Location to Route:

1. compareTarget can be a Location
   https://github.com/vuejs/vue-router/blob/next/src/components/link.js#L31
2. and it's passed into isSameRoute whose second param is type ?Route (which means Route | void | null if I understood correctly), but flow didn't give a warning
   https://github.com/vuejs/vue-router/blob/next/src/components/link.js#L33

[posva]

Location can be an empty object because everything is optional in it.
Is there a way on flow to say a Location must have either a name or a path?
I think using multiple types and then saying location is one of them can work

[fnlctrl]

@posva A naive approach can be

declare type Location = {
  _normalized?: boolean;
  name?: string;
  path: string;
  hash?: string;
  query?: StringHash;
  params?: StringHash;
} | {
  _normalized?: boolean;
  name: string;
  path?: string;
  hash?: string;
  query?: StringHash;
  params?: StringHash;
}

But it seems clumsy... I'll dig up flow's documents

[posva]

I think we are looking for this https://flowtype.org/docs/disjoint-unions.html#_

edit: which is actually what you did xD

It'd be great to combine unions with intersections

[fnlctrl]

@posva Tried that, but flow still doesn't give warnings : (
and even if we make it give warnings, we still have to fix passing Location to a function that requires Route..

[yyx990803]

The root cause was because components/link was not flow-enabled... I've fixed it and made some adjustments in 270683f