One case of careless opDispatch :)

[Dmitry Olshansky]

As a practical habit, once I stumble upon a very tricky error, I usually 
share the valuable knowledge of "when you do this ... and get that ... 
it's probably because ... "
Damn, sometimes they can even become cool quizzes...
So to warn those oblivious to the dangers of opDispatch, here is my the 
yesterday nightmare, the striped down code below.

import std.algorithm;
import std.array;

class Widget {
     string _name;
     Widget[] _children;
     this(in string name){
         _name = name.idup;
     }
     Widget opDispatch(string nm)(){
         auto r = find!((Widget c){ return c._name == nm; })(_children);
         return r.front();
     }
}

void main(){
     Widget g = new Widget("G");
     Widget warr[] = [new Widget("W"),g];
     find(warr,g);
}

produces:

Error    1    Error: template std.algorithm.startsWith(alias pred = "a 
== b",Range,Ranges...) if (isInputRange!(Range) && Ranges.length > 0 && 
is(typeof(binaryFun!(pred)(doesThisStart.front,withOneOfThese[0].front)))) 
startsWith(alias pred = "a == b",Range,Ranges...) if 
(isInputRange!(Range) && Ranges.length > 0 && 
is(typeof(binaryFun!(pred)(doesThisStart.front,withOneOfThese[0].front)))) 
matches more than one template declaration, 
C:\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(1892):startsWith(alias 
pred = "a == b",Range,Ranges...) if (isInputRange!(Range) && 
Ranges.length > 0 && 
is(typeof(binaryFun!(pred)(doesThisStart.front,withOneOfThese[0].front)))) 
and 
C:\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(1980):startsWith(alias 
pred = "a == b",Range,Elements...) if (isInputRange!(Range) && 
Elements.length > 0 && 
is(typeof(binaryFun!(pred)(doesThisStart.front,withOneOfThese[0]))))    
C:\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d    1488

The tricky part is that *any* class with unconstrained (or loosely 
constrained) opDispatch is also a Range, and at least a bidirectional 
one, since it "provides" all the primitives: front, popFront etc.
In fact such classes could penetrate almost any attempts at C++ 
trait-like stuff  and should be avoided.

The moral: unconstrainted opDispatch == TROUBLE.
Hope that helps!

P.S. Strangely enough, that problem haven't showed up until update to 
2.047 release, so it's probably detonated by some changes to Phobos. I 
guess better sooner than later.

-- 
Dmitry Olshansky