The expression being assigned to '[something] must be constant when using integers inside strings

I've stumbled upon a very funny exception today. Basically I was creating a constant string from adding some other constant strings to each other. And it worked. The moment I added an integer, though, I got The expression being assigned to 'Program.x2' must be constant. The code that generated this error is simple:

const string x2 = "string" + 2;

Note that

const string x2 = "string" + "2";

is perfectly valid. Got the same result when using VS2010 and VS2015, so it's not a compiler bug, it's intended behavior.

So, what's going on? Well, my code transforms behind the scenes into

const string x2 = "string" + 2.ToString();

which is not constant because of ToString!

The only way to solve it was to declare the numeric constant as string as well.