Why conversion to String^ fails?

Question

In the following code two calls to conversion to String^ fails:

ref class s sealed
{
	void fun(String ^)
	{
	}

	void foo()
	{
		fun("this"); //OK

		fun(ref new String("this")); //fails, cannot convert parameter 1 from 'const char [5]' to 'const wchar_t *'

		fun(L"this"); //OK
		auto str = L"this";
		fun(str); //fails, cannot convert parameter 1 from 'const wchar_t *' to 'Platform::String ^'
	}
};

I am not able to understand why it fails.

1. fun("this") is oK but not fun(ref new String("this"))? How literal "this" is converted to String ^ while calling fun("this")?

2. In the second case what is the difference between fun(L"this") and fun(str)? In both cases, fun is called with const wchar_t *, then why it fails in the latter case?