Mine

// from another unit:
// lua_CFunction      = function(l: plua_State): integer; stdcall;

type
  TGVLuaFunction = lua_Cfunction;

  TGVLua = class
  [...]
    function  PopStackFunction(): TGVLuaFunction;
    function  GetFunction(aStructStackIndex: Integer; aFieldName: String): TGVLuaFunction;
  [...]
  end;

function  TGVLua.PopStackFunction(): TGVLuaFunction;
begin
  Result := lua_tocfunction(FLua.FState, -1);
  // lua_tocfunction() returns a value of type lua_CFunction (TGVLuaFunction)
  Pop(1);
end;

function TGVLua.GetFunction(aStructStackIndex: Integer; aFieldName: String): TGVLuaFunction;
begin
  lua_pushstring(FState, PChar(aFieldName));
  lua_gettable(FState, aStructStackIndex);
  // the above sets up the Lua stack with the value, and the below retrieves it.
  Result := PopStackFunction;
  // [Error]: Incompatible types: 'Calling conventions differ' on PopStackFunction.
end;