template instantiation --- having trouble therewith

[John Colvin]

On Thursday, 5 September 2013 at 09:33:00 UTC, John Colvin wrote:
> On Thursday, 5 September 2013 at 09:11:06 UTC, John Colvin 
> wrote:
>> On Thursday, 5 September 2013 at 09:00:27 UTC, John Colvin 
>> wrote:
>>> On Tuesday, 3 September 2013 at 11:51:37 UTC, Carl Sturtivant 
>>> wrote:
>>>>>
>>>>> I'm confused as to what you're trying to do... your example 
>>>>> code is equivalent to
>>>>>
>>>>> import std.stdio;
>>>>> int x = 3;
>>>>> int scale( int s) { return x * s; }
>>>>> auto f = &scale;
>>>>> writeln( f(7) );
>>>>
>>>> No it isn't according to dmd.
>>>>
>>>> My code is a minimal piece that produces the same error as 
>>>> some real code. The higher order generic function muddle in 
>>>> the real code is supposed to transform one delegate into 
>>>> another, but I still get the template problem if muddle is 
>>>> the identity function (given here).
>>>>
>>>> My example code isn't equivalent to the above according to 
>>>> the compiler. Why is that? And how can I make it work?
>>>
>>> Ok, that makes more sense.
>>>
>>> The reason why it doesn't work is that you're effectively 
>>> asking the compiler to work backwards from {the type of the 
>>> delegate passed to muddle} to {the type parameters that would 
>>> have to be used in Dptr to generate that delegate type}.
>>> I'm no expert on type-deduction algorithms, but I seriously 
>>> doubt that's a solvable problem in the general case, 
>>> especially considering that the definition of Dptr could 
>>> contain string mixins etc.   In the general case, all code is 
>>> forwards-only.
>>>
>>> Anyhow, it's easy to work around:
>>>
>>> import std.traits : ReturnType, ParameterTypeTuple;
>>>
>>> template Dptr( T, U...) {
>>> 	alias T delegate( U args) Dptr;
>>> }
>>>
>>> auto muddle( DT)( DT f) {
>>> 	alias T = ReturnType!f;
>>> 	alias U = ParameterTypeTuple!f;
>>> 	//use T and U
>>> 	return f; //or make another delegate in real code
>>> }
>>>
>>>
>>> unittest {
>>> 	import std.stdio;
>>> 	int x = 3;
>>> 	int scale( int s) { return x * s; }
>>> 	Dptr!(int,int) f = muddle( &scale);
>>> 	writeln( f(7));
>>> }
>>
>> Having said that, in the case where you explicitly set the 
>> template parameters to muddle I think it *should* work and is 
>> probably a bug (or at least a simple enhancement) that it 
>> doesn't.
>
> I filed a bug report for it
> http://d.puremagic.com/issues/show_bug.cgi?id=10969

and only 26.5 mins after posting the bug, there's a pull request 
to fix it: https://github.com/D-Programming-Language/dmd/pull/2526

That's what I call service :p