whihc constructor does this call? :'( this part of the language confuses me.

Yes, we did not end up in a good spot here with structs. Not sure if this can be fixed, probably not. This will call the explicitly defined parameterless ctor. The one with optional parameters is different one. That's actually why I'm defining the parameterless one so that one does not end up in a trap of calling new T() and ending up with default(T) instead of the default values.

Also, something like this would be useful:

readonly record struct R(optional T X = <expr>); // T must be non-nullable type

which would translate to:

readonly struct R
{
  public readonly T X;
  R(T? X = default) { this.X = X ?? <expr>; }
}

where <expr> is an arbitrary expression, not limited to constants.