Peek definition doesn't work on source generated code

[svick]

Version Used: VS 17.5.0 Preview 1.0

Steps to Reproduce:

1. Create a console application that uses GeneratedRegex:

   using System.Text.RegularExpressions;
   
   partial class Program
   {
       static void Main() { }
   
       [GeneratedRegex(".")]
       private static partial Regex Regex();
   }

2. Right click on the partial method and select Peek Definition.

Expected Behavior: Peek Definition shows the generated code.

Actual Behavior:

Peek Definition says that it cannot open the file:

Note that Go To Definition works just fine: