Why is Xcode 7.3 showing a slash through the app icon when attempting to install on Mac OS X 10.10.5 (Yosemite)
It turns out that Xcode 7.3 requires Mac OS X 10.11 (El Capitan) or higher to run see screenshot below from the Mac App Store but the same idea for either install method.

Oh well. Been nice knowing you Yosemite!

As one can see from this question I posted a little while back I’m fairly cautious with my core OS upgrades so I jumped to Mac OS X 10.10 when El Capitan was released just to not be too far behind the curve. Now, I guess I need to finally make the jump to El Capitan!