methods do not bind templates via alias parameter

[dlangBugzillaToGithub]

Vladimir Panteleev (@CyberShadow) reported this on 2014-02-23T01:49:08Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=12230

Description

///////////////////// test.d ////////////////////
import std.stdio;

static template T(alias a)
{
    void foo() { writeln(a); }
}

struct S
{
    int i = 1;
    @property int p() { return 2; }

    alias ti = T!i; // OK
    alias tp = T!p; // Error
}
/////////////////////////////////////////////////

The compiler rejects the above code, with the error:
test.d(5,23): Error: need 'this' for 'p' of type '@property int()'

The problem can be worked around by adding an anchor parameter to the template. This correctly sets the "this" type for the template:

//////////////////// test2.d ////////////////////
static template T(alias a, alias anchor = Object)
{
    void foo() { writeln(a); }
}

struct S
{
    int i = 1;
    @property int p() { return 2; }

    alias ti = T!(i);    // bound to S implicitly
    alias tp = T!(p, i); // bound to S via anchor
}
/////////////////////////////////////////////////

[dlangBugzillaToGithub]

dlang-bugzilla (@CyberShadow) commented on 2014-02-23T01:51:47Z

Actually, this refers to all non-static methods, not just properties.

[dlangBugzillaToGithub]

dlang-bugzilla (@CyberShadow) commented on 2014-03-02T12:32:27Z

https://github.com/D-Programming-Language/dmd/pull/3345

[dlangBugzillaToGithub]

k.hara.pg commented on 2016-03-15T15:19:33Z

(In reply to Vladimir Panteleev from comment #0)
> ///////////////////// test.d ////////////////////
> import std.stdio;
> 
> static template T(alias a)
> {
>     void foo() { writeln(a); }
> }
> 
> struct S
> {
>     int i = 1;
>     @property int p() { return 2; }
> 
>     alias ti = T!i; // OK
>     alias tp = T!p; // Error
> }
> /////////////////////////////////////////////////
> 
> The compiler rejects the above code, with the error:
> test.d(5,23): Error: need 'this' for 'p' of type '@property int()'

From the long discussion in issue 11946, the static template T cannot get any context even if the aliased symbol a is an instance member.

[dlangBugzillaToGithub]

k.hara.pg commented on 2016-03-15T15:35:12Z

(In reply to Kenji Hara from comment #3)
> From the long discussion in issue 11946, the static template T cannot get
> any context even if the aliased symbol a is an instance member.

Or... am I misunderstanding something?

[dlangBugzillaToGithub]

dlang-bugzilla (@CyberShadow) commented on 2016-03-15T16:19:13Z

Well, for one thing, there is the inconsistence - if fields bind context, why don't methods?

In much broader terms, I really really wish we'd have explicit control over alias context binding, at the instantiation site.

Template aliases can allow for some very powerful things with a few small tweaks. I've written a serialization framework with almost zero overhead which depends on the compiler patches I wrote:

https://github.com/CyberShadow/ae/tree/master/utils/serialization

However, the compiler patches were not accepted so this is mostly defunct.

For personal use I'm still using a patched compiler with PR #3884 reverted. I realize this is hypocritical of me because I argued a lot in favor of that PR (as a fix to code breakage), however I have come to depend on the behavior that PR reverted in my allocators library.

I would like to some day make a case for these small language improvements, but I feel like it would be more wasted work. I almost went mad just from attempting to implement __traits(child) correctly.