isVariantN - is expression doesn't match

[Piotr Szturmaj]

I wrote simple template to test VariantN type (and thus Algebraic and 
Variant).

template isVariantN(T)
{
	static if (is(T X == VariantN!(N, Types), uint N, Types...))
		enum isVariantN = true;
	else
		enum isVariantN = false;
}

but testing against Variant yields false:
static assert(isVariantN!(Variant)); // fail

Variant is declared as an alias to VariantN:
alias VariantN!(maxSize!(creal, char[], void delegate())) Variant;

which is equivalent to VariantN!(20u) on 32 bit DMD 2.052:
static assert(is(Variant == VariantN!(20u))); // pass

Strange thing is when I set maxSize other than 20u then isVariantN 
yields true:

static assert(isVariantN!(VariantN!(21u))); // pass
static assert(isVariantN!(VariantN!(20))); // pass, 20 as int
static assert(isVariantN!(VariantN!(20u))); // fail, same as Variant

If I add at least one type then it passes even for 20u maxSize.

static assert(isVariantN!(VariantN!(21u, int))); // pass
static assert(isVariantN!(VariantN!(20, int))); // pass, 20 as int
static assert(isVariantN!(VariantN!(20u, int))); // pass

What is going on here?

I also have question regarding integral type aliases such as size_t. For 
example I can't use size_t in is expression:

template isVariantN(T)
{
	static if (is(T X == VariantN!(N, Types), size_t N, Types...))
		enum isVariantN = true;
	else
		enum isVariantN = false;
}

Error: undefined identifier size_t

TIA