map!(char)(string) problem

[David Held via Digitalmars-d-learn]

import std.algorithm;

int toInt(char c) { return 1; }

void main()
{
     map!(a => toInt(a))("hello");
}

Can someone please explain why I get this:

Bug.d(10): Error: function Bug.toInt (char c) is not callable using 
argument types (dchar)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
D:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(425): Error: 
template instance Bug.main.__lambda1!dchar error instantiating
D:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(411): 
instantiated from here: MapResult!(__lambda1, string)
Bug.d(10):        instantiated from here: map!string
D:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(411): Error: 
template instance Bug.main.MapResult!(__lambda1, string) error instantiating
Bug.d(10):        instantiated from here: map!string
Bug.d(10): Error: template instance Bug.main.map!((a) => 
toInt(a)).map!string error instantiating

I thought that string == immutable char[], but this implies that it is 
getting inferred as dchar[], I guess.

Dave