Error with constraints on a templated fuction

[Jeremy DeHaan via Digitalmars-d-learn]

I've done things like this before with traits and I figured that 
this way should work as well, but it gives me errors instead. 
Perhaps someone can point out my flaws.

immutable(T)[] toString(T)(const(T)* str)
	if(typeof(T) is dchar)//this is where the error is
{
	return str[0..strlen(str)].idup; //I have strlen defined for 
each *string
}

I was going to add some || sections for the other string types, 
but this one won't even compile.

src/dsfml/system/string.d(34): Error: found ')' when expecting 
'.' following dchar
src/dsfml/system/string.d(35): Error: found '{' when expecting 
identifier following 'dchar.'
src/dsfml/system/string.d(36): Error: found 'return' when 
expecting ')'
src/dsfml/system/string.d(36): Error: semicolon expected 
following function declaration
src/dsfml/system/string.d(36): Error: no identifier for 
declarator str[0 .. strlen(str)]
src/dsfml/system/string.d(36): Error: no identifier for 
declarator .idup
src/dsfml/system/string.d(37): Error: unrecognized declaration


It compiles if I remove the 'if(typeof(T) is dchar)' section. Any 
thoughts?