[Issue 12223] New: __traits(getMember,...) needed for aliases

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=12223

           Summary: __traits(getMember,...) needed for aliases
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: thecybershadow at gmail.com


--- Comment #0 from Vladimir Panteleev <thecybershadow at gmail.com> 2014-02-22 08:21:27 EET ---
Currently, "x.y" does two things:

1. Looks up "y" in the symbol scope of "x"
2. Interprets "y" with its "this" set to "x"

Sometimes, it is useful to separate these two things.
One can already do 1 without 2 using alias:

    alias y = x.y;

However, there is no way to do 2 without 1.
The closest thing we have is __traits(getMember, x, y) - however, it requires
that y is a string, and does not work if it is an alias.

Example:
////////////////////////////////// test.d //////////////////////////////////
static template T(alias v)
{
    void set() { v = 42; }
}

struct S
{
    int i;
    alias t = T!i;
}

void main()
{
    alias f = S.t.set;

    // ...

    S s;

    // Goal: call f() with "this" set to s, like this:
    s.t.set();
    // ... (but without referring to "t" or "set" directly)

    //s.f();                        // NG

    //with (s) f();                 // NG

    //struct W { S s; alias s this; alias wf = f; }
    //W w = W(s); w.wf();           // NG (Issue 12222)

    //__traits(getMember, s, f)();  // NG

    //__traits(getMember, 
    //  __traits(getMember, s, __traits(identifier, __traits(parent, f))),
    //  __traits(identifier, f))(); // NG
}
////////////////////////////////////////////////////////////////////////////

I propose adding a __traits(child, x, y), since it will be the reverse
operation of __traits(parent, y).

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