std.format and uninitialized elements in CTFE

[mipri]

On Thursday, 5 December 2019 at 19:56:35 UTC, kinke wrote:
> there's no such thing as uninitialized struct/class *members*, 
> there are only uninitialized whole struct instances.

Is that true in CTFE?

Consider:

   import std.format;

   struct Foo
   {
       int a = void;
       int b;
   }

   static x = format("%s", Foo().a);
   static y = format("%s", Foo().b);

The second format succeeds on its own.
The first fails with

   Error: cannot read uninitialized variable a in CTFE