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 | Posted by ag0aep6g@gmail.com |  Permalink Reply |
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ag0aep6g@gmail.com 
| https://issues.dlang.org/show_bug.cgi?id=16222
ag0aep6g@gmail.com changed:
What |Removed |Added
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CC| |ag0aep6g@gmail.com
--- Comment #1 from ag0aep6g@gmail.com ---
(In reply to Ketmar Dark from comment #0)
> consider the following code:
>
> auto test(T : const(char)[]) (T s) { return s.length; }
> void main () { assert(test(null) == 0); }
>
> compiling this gives the following error: "no property 'length' for type
> 'typeof(null)'"
>
> while i see why it complains, i think compiler should convert `null` to empty slice here, as i clearly specified the type i want to accept. sure, i can workaround this with explicit template constrain... but this is what i wanted to avoid in the first place!
The spec [1] isn't clear on this.
The currently implemented behavior of a constraint `T : Foo` is to restrict T to those types that implicitly convert to Foo, but T is still set as usual (from an explicit argument or from IFTI).
The behavior you suggest would make it so that T is always set to Foo. Obviously, this would be a breaking change.
Both approaches are compatible with the examples in the spec, as far as I see.
In your example, you could of course just take a const(char)[] directly, without any template parameters. So that isn't a good motivating example to change the behavior.
In favor of the current behavior there is std.exception.assertThrown [2], for example. It uses a specialization to restrict T to Throwable, but it uses the (more restricted) passed T in its checks.
> also, another bug with the same `test`:
> assert(test([]) == 0);
>
> this time compiler says: "
> template z01.test cannot deduce function from argument types !()(void[]),
> candidates are..."
>
> i think compiler should convert empty slice to the `immutable(char)[]` here.
Agreed. The empty array literal [] should behave like null and the empty string
literal "" here. I guess this means we need a typeof([]) that implicitly
converts to any T[] (unlike void[]).
[1] http://dlang.org/spec/template.html#parameters_specialization [2] http://dlang.org/phobos/std_exception.html#.assertThrown
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