Why this fails when using unittest?

[Machine Code]

outside an unittest, this compiles fine:

	struct A
	{
		enum A foo = "hehe";
		this(string a) { m_a = a; }
		alias m_a this;
		string m_a;
	}

but if you wrap this a unittest {} and compile with
> dmd -unittest -run foo.d

this give the following error:

> cannot implicitly convert expression "hehe" of type string to A

Why does inside a unittest it doesn't work? the constructor 
this(string) {} seems to get disabled.