D function templates and type inference

Question

Consider the following code:

module ftwr;

import std.regex;
import std.stdio;
import std.conv;
import std.traits;

S consume (S) (ref S data, Regex ! ( Unqual!(typeof(S.init[0])) ) rg)
{
    writeln (typeid(Unqual!(typeof(S.init[0]))));

    auto m = match(data, rg);
    return m.hit;
}

void main()
{
    auto data = "binary large object";
    auto rx = regex(".*");
    consume (data, rx); // this line is mentioned in the error message
}

Now, I expect the compiler to infer that consume is to be instantiated as

string consume!(string)(string, Regex!(char))

but that doesn't seem to happen. The errors are as follows:

func_template_with_regex.d(24): Error: template ftwr.consume(S) does not match any function template declaration
func_template_with_regex.d(24): Error: template ftwr.consume(S) cannot deduce template function from argument types !()(string,Regex!(char))

and I see that the parameter types are correct... I've tried some variations of function signature, like:

S consume (S) (Regex ! ( Unqual!(typeof(S.init[0])) ) rg, ref S data)

which doesn't compile also (the idea was to change the order of arguments), and

immutable(S)[] consume (S) (Regex ! ( S ) rg, ref immutable(S)[] data)

which compiles and infers the types alright. If I specify the type explicitly in the call, i.e.

consume!string(data, rx);

it also compiles and the debug writeln prints char, just as expected. Am I missing something in the inference rules, or I've just hit a bug in the compiler?

Oh yes:

$ dmd -v
DMD64 D Compiler v2.053
...

Answer 1

I can't say if it's a bug, but here's a workaround which doesn't force you to specify the type or change the order of arguments. Change consume's signature to:

S consume (S, U) (ref S data, Regex!U rg) if (is(U == Unqual!(typeof(S.init[0]))))