[Issue 1807] ENHANCEMENT: Let IFTI "see through" templates to simple aliases

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https://issues.dlang.org/show_bug.cgi?id=1807

kdevel  changed:

   What|Removed |Added

 CC||kde...@vogtner.de

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[Issue 1807] ENHANCEMENT: Let IFTI "see through" templates to simple aliases

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FeepingCreature  changed:

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 CC||default_357-l...@yahoo.de

--- Comment #12 from FeepingCreature  ---
*** Issue 23798 has been marked as a duplicate of this issue. ***

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[Issue 1807] ENHANCEMENT: Let IFTI "see through" templates to simple aliases

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https://issues.dlang.org/show_bug.cgi?id=1807

Iain Buclaw  changed:

   What|Removed |Added

   Priority|P2  |P4

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[Issue 1807] ENHANCEMENT: Let IFTI "see through" templates to simple aliases

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https://issues.dlang.org/show_bug.cgi?id=1807

--- Comment #11 from John Hall  ---
Creating a new type with an alias this'ed parameter is similar conceptually to
a template alias. The new type is implicitly convertible to the target of the
alias this and can be used wherever that original type was used. However, since
it is a separate type, it can't called called where the alias is used. 

```d
struct TemplateType(T) {}

//alias TemplateAlias(T) = TemplateType!T;
struct TemplateAlias(T) {
TemplateType!T x;
alias x this;
}

void templateFunction(T)(TemplateType!T arg) {}
void templateAliasFunction(T)(TemplateAlias!T arg) {}

void main()
{
TemplateType!int inst;
TemplateAlias!int ainst;

templateFunction(inst);
templateFunction(ainst);
templateAliasFunction(inst); //doesn't compile
templateAliasFunction(ainst); 
}
```

Perhaps what is needed is some kind of mechanism like alias this...not in the
sense of implicit conversion but in terms the programmer having the tools to
tell the compiler what to do.

The idea would be that your template alias would include an additional template
(with some rudimentary sketch below), call it `opResolveAlias`, that would let
the compiler know the relationships going in reverse (so instead of from the
alias to the alias target, this would be going from the alias target to the
alias). The compiler could then make use of that information to make sure the
function call can properly occur (with type inference as needed). 

So for instance, if you have a function that accepts a `TemplateAlias!T`
parameter, but pass in a `TemplateType!int` parameter, then currently the
compiler doesn't look through the `TemplateAlias!T` to see that
`TemplateType!int` is the same. However, if the compiler knows that a
`TemplateType!int` is the same as `TemplateAlias!int`, then it would be better
able to proceed. 

```d
template TemplateAlias(T) {
alias TemplateAlias = TemplateType!T;

template opResolveAlias(alias U)
if(is(U == TemplateType!V, V))
{
alias opResolveAlias = TemplateAlias!V;
}
}
```

For simple cases like this, the compiler could generate its own
`opResolveAlias`, but in the more general case it would be up to the user to
provide it if it is important to them.

Issue 16486 has an example of a more complicated template alias [1] that should
still be able to have this approach applied:

```d
template TestAlias(T)
{
static if (is(T == float)) alias TestAlias = TestType!int;
else alias TestAlias = string;

// new part:
template opResolveAlias(alias U)
{
static if (is(U == TestType!int)) {
alias opResolveAlias = TestAlias!float;
} else static if (is(U == string)) {
alias opResolveAlias = TestAlias!int; //could put anything there
that's not float...not the most elegant
}
}
}
```

[1] https://issues.dlang.org/show_bug.cgi?id=16486#c1

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[Issue 1807] ENHANCEMENT: Let IFTI "see through" templates to simple aliases

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https://issues.dlang.org/show_bug.cgi?id=1807

John Hall  changed:

   What|Removed |Added

 CC||john.michael.h...@gmail.com

--- Comment #10 from John Hall  ---
(In reply to Walter Bright from comment #9)
> Proposed DIP:
> 
> https://github.com/dlang/DIPs/blob/master/DIPs/other/DIP1023.md
> 
> Proposed Implementation:
> 
> https://github.com/dlang/dmd/pull/9778

DIP 1023 has been postponed.

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[Issue 1807] ENHANCEMENT: Let IFTI "see through" templates to simple aliases

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ag0aep6g  changed:

   What|Removed |Added

 CC||ag0ae...@gmail.com
Summary|Wad3|ENHANCEMENT: Let IFTI "see
   ||through" templates to
   ||simple aliases

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[Issue 1807] ENHANCEMENT: Let IFTI "see through" templates to simple aliases

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https://issues.dlang.org/show_bug.cgi?id=1807

--- Comment #9 from Walter Bright  ---
Proposed DIP:

https://github.com/dlang/DIPs/blob/master/DIPs/other/DIP1023.md

Proposed Implementation:

https://github.com/dlang/dmd/pull/9778

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[Issue 1807] ENHANCEMENT: Let IFTI "see through" templates to simple aliases

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https://issues.dlang.org/show_bug.cgi?id=1807

Walter Bright  changed:

   What|Removed |Added

   See Also||https://issues.dlang.org/sh
   ||ow_bug.cgi?id=1653,
   ||https://issues.dlang.org/sh
   ||ow_bug.cgi?id=1942,
   ||https://issues.dlang.org/sh
   ||ow_bug.cgi?id=3904,
   ||https://issues.dlang.org/sh
   ||ow_bug.cgi?id=7529,
   ||https://issues.dlang.org/sh
   ||ow_bug.cgi?id=10884,
   ||https://issues.dlang.org/sh
   ||ow_bug.cgi?id=16465,
   ||https://issues.dlang.org/sh
   ||ow_bug.cgi?id=16486

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[Issue 1807] ENHANCEMENT: Let IFTI "see through" templates to simple aliases

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https://issues.dlang.org/show_bug.cgi?id=1807

--- Comment #8 from RazvanN  ---
*** Issue 1942 has been marked as a duplicate of this issue. ***

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[Issue 1807] ENHANCEMENT: Let IFTI "see through" templates to simple aliases

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anonymous4  changed:

   What|Removed |Added

   Keywords||spec
   Hardware|x86 |All
 OS|Linux   |All

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[Issue 1807] ENHANCEMENT: Let IFTI "see through" templates to simple aliases

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https://issues.dlang.org/show_bug.cgi?id=1807

--- Comment #7 from ag0aep6g  ---
*** Issue 10884 has been marked as a duplicate of this issue. ***

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[Issue 1807] ENHANCEMENT: Let IFTI see through templates to simple aliases

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http://d.puremagic.com/issues/show_bug.cgi?id=1807


d...@dawgfoto.de changed:

   What|Removed |Added

 CC||d...@dawgfoto.de


--- Comment #6 from d...@dawgfoto.de 2012-02-18 09:40:40 PST ---
One issue with this is that treating certain kinds of templates
different that others will make it very difficult to understand
why matching might fail.

If we want to have alias templates to behave different than plain
templates they should have a language construct that enforces the
limitations and resembles struct templates.

alias(T) void delegate(T*) wrap;
expands to:
template wrap(T) { alias void delegate(T*) wrap; }

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[Issue 1807] ENHANCEMENT: Let IFTI see through templates to simple aliases

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timon.g...@gmx.ch changed:

   What|Removed |Added

 CC||kyfo...@gmail.com


--- Comment #5 from timon.g...@gmx.ch 2012-02-17 14:20:38 PST ---
*** Issue 7529 has been marked as a duplicate of this issue. ***

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[Issue 1807] ENHANCEMENT: Let IFTI see through templates to simple aliases

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Steven Schveighoffer schvei...@yahoo.com changed:

   What|Removed |Added

 CC||bearophile_h...@eml.cc


--- Comment #4 from Steven Schveighoffer schvei...@yahoo.com 2010-03-09 
04:15:46 PST ---
*** Issue 3904 has been marked as a duplicate of this issue. ***

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