GPRBuild: Reuse Default_Switches from an imported project

[Jere <jhb.chat@gmail.com>]

I have an embedded library X that I want to import into 
a project Y.  In project X I have (among other things)

   for Runtime ("Ada") use Project'Project_Dir & "/runtime";

   for Target use "arm-eabi";

   package Compiler is

      case Build_Mode is

         when "debug" =>
            for Default_Switches ("ada") use ("-O0", "-gnatwa.X", "-gnatQ", "-gnat12", "-g");

         when "release" =>
            for Default_Switches ("ada") use ("-O2", "-gnatQ", "-gnatw.X", "-gnat12", "-gnatn", "-ffunction-sections", "-fdata-sections");
      end case;
      
   end Compiler;


In project Y, I with project X:

   with "path/X.gpr"

and I am able to do the following just fine:

   for Runtime ("Ada") use X'Runtime("Ada");

But when I try to use the Default_Switches in the same manner
it gives me errors

   y.gpr:23:46: wrong expression kind for attribute "default_switches"
   y.gpr:23:64: unknown variable "Default_Switches"

For reference I am trying the syntax:

   package Compiler is

      case Build_Mode is

         when "debug" =>
            for Default_Switches ("ada") use X.Compiler.Default_Switches("ada");

         when "release" =>
            for Default_Switches ("ada") use X.Compiler.Default_Switches("ada");
            
      end case;

   end Compiler;

I even tried putting them inside a set of parenthesis, but with no luck.
I am guessing I need some way to indicate which case the default switches
are from, but am unsure.  It may not even be possible.  Anyone know
how to do this or if it possible