How to obtain GUID of interface passed as a generic parameter in Delphi?

In the following, the method Obtain() works, but the GetAs() method would be neater for callers.

However, I can't figure out how to pass an interface as a generic parameter and obtain its GUID.

Declaration:

  TInterfaceRegistry = class
  private
    fRegistry: TDictionary<TGUID, IInterface>;
  public
    ...
    procedure Register(const IID: TGUID; IntfObj: IInterface; const Replace: Boolean = False);
    function  Obtain(const IID: TGUID; out IntfObj): Boolean;
    function  GetAs<I: IInterface>(): I;
  end;

Implementation:

function TInterfaceRegistry.Obtain(const IID: TGUID; out IntfObj): Boolean;
var
  Found: IInterface;
begin
  Result  := fRegistry.TryGetValue(IID, Found);
  if  Result  then
    if  not Supports(Found, IID, IntfObj) then
      raise EBatSoft.Create(SEBatSoftBug);
end;

function  TInterfaceRegistry.GetAs<I>(): I;
begin
  if  not Obtain(I, Result) then
    raise EBatSoft.Create(SEDoesNotImplement);
end;

Calling it looks like this...

  IntfReg.Register(IMyIntf, TMyImpl.Create);

Getting implementation:

var
  Intf: IMyIntf;
begin
  // Less type-safe (can pass anything into 2nd parameter)
  if IntfReg.Obtain(IMyIntf, Intf)  then
    ...
  // Fully type-safe, and simple
  Intf := IntfReg.GetAs<IMyIntf>();