I have an iOS app. I am thinking about using an mdash in the app name. In other words, calling the app
Foo — Bar
I am wondering if that would have any impact on the discoverability of my app. In other words, would using a character that is not on the keyboard make people less like it in searches?
Apple not only use your App name but also use keywords and your company name as search elements. Read this article:
Algorithm
The algorithm matches search terms to keywords specified in the application name, keywords field, and company name. The application name allows for 255 characters while the keywords field only allows for 100 characters. The fact that Apple gives you more space for your app name means more and more developers are stuffing it with keywords.
http://www.techrepublic.com/blog/ios-app-builder/itunes-app-store-search-optimization-tips/438
So the discoverability of your app mainly stands on your name and keywords, also stands on App screenshots since users won't even open your App's introduction page if you used an ugly screenshot. In my opinion, add a "dash" in your App name won't help or impact much, Apple's algorithm might even ignore "dash" & spaces in App name(that's just my guess, but reasonable for a search engine). Keywords and screenshots are critical.
PS: Don't try to add keywords not related with your App(e.g. "twitter" or "google") to increase discoverability. You might be punished.
PS2: Don't try to fill your App name with many keywords(>3 in my opinion). It will only make users feel confused and get away from it.