Using decodeFront with a generalised input range

[Vinay Sajip]

According to the std.utf documentation,

> decode will only work with strings and random access ranges of 
> code units with length and slicing, whereas decodeFront will 
> work with any input range of code units.

However, I can't seem to get such a usage to compile: the 
following code

import std.range;
import std.utf;

void somefn(InputRange!(ubyte) r) {
     r.decodeFront!(No.useReplacementDchar, dchar)();
}

gives a compilation error:

onlineapp.d(5): Error: template std.utf.decodeFront cannot deduce 
function from argument types !(cast(Flag)false, 
dchar)(InputRange!ubyte), candidates are:
/dlang/dmd/linux/bin64/../../src/phobos/std/utf.d(1176):        
std.utf.decodeFront(Flag useReplacementDchar = 
No.useReplacementDchar, S)(ref S str, out size_t numCodeUnits) if 
(!isSomeString!S && isInputRange!S && isSomeChar!(ElementType!S))
/dlang/dmd/linux/bin64/../../src/phobos/std/utf.d(1214):        
std.utf.decodeFront(Flag useReplacementDchar = 
No.useReplacementDchar, S)(ref S str, out size_t numCodeUnits) if 
(isSomeString!S)
/dlang/dmd/linux/bin64/../../src/phobos/std/utf.d(1243):        
std.utf.decodeFront(Flag useReplacementDchar = 
No.useReplacementDchar, S)(ref S str) if (isInputRange!S && 
isSomeChar!(ElementType!S))

I'm not sure what's wrong here. Also, the information in the 
error message seems to indicate that decodeFront isn't expecting 
to work on a generic InputRange interface, but rather a subset 
which comes from an actual string. Have I read that wrong? I get 
the same error with leaving out the useReplacementDchar flag, 
i.e. `r.decodeFront!(dchar)()`