A tweet from Microsoft engineer @jacobrossi confirms that Windows 10 users will shortly be able to specify Google as the default search provider in Edge, Microsoft's new Windows 10-only browser. Per Rossi:
Thanks to our friends at Google for rolling out OpenSearch 1.1 support for Microsoft Edge
Apparently Google has always supported OpenSearch 1.0, which was released in March 2005 by a division of Amazon. For unknown reasons, Google never made the changes necessary to move to OpenSearch 1.1, which went out as a draft spec in December 2005.
Until Windows 10 Technical Preview build 10058, Edge accepted Google as a default search provider. However, starting with build 10058, Edge only accepted Bing.
Making the modification couldn't have been too difficult. There's a Web page, orthodox.io/edge, that adds Open Search 1.1 to the Google, Reddit, Yahoo, Ask, and Yandex search engines. According to the site's creator, Sandeep Bansal, "The site doesn't act as a proxy for your search calls, it simply adds the search engine directly in the browser."
At this moment, my various versions of Edge (in various versions of Windows 10) don't find Google as a search engine. I'm assured that, with time, Google will show up everywhere.
If you're running Windows 10 Technical Preview build 10058 or later, open Edge, then click the ellipses button in the upper right corner and choose <Settings>. At the bottom, tap or click <View advanced settings>. Scroll way down to the box marked <Search in the address bar with>. Click the down arrow next to Bing and choose <Add new>. Under <Choose one>, you should see Google ... sooner or later. Click on <Google>, then click <Add as default>.
Funny, but I don't see any announcement from Google about its new-found search companion.