[Issue 15634] new __traits request: isLvalue

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=15634

Stanislav Blinov <stanislav.blinov at gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |stanislav.blinov at gmail.com

--- Comment #1 from Stanislav Blinov <stanislav.blinov at gmail.com> ---
Stumbled on this via a forum thread (four years later, yay), and honestly I
find this request rather strange.

There is a trait that provides such information for functions - the
getFunctionAttributes. It already existed at the time of this issue report, and
is even touched upon in aforementioned forum thread, albeit in Phobos disguise.

However, the "some alias/member" may be (among others):
- an overloaded function
- a function template with auto ref parameters

In both of those cases, a __trait won't be able to provide the information
required, since symbol alone is simply insufficient. Overloads would need to be
tested one by one, carefully matching arguments; templates would have to be
instantiated, again carefully matching arguments. Consider:

struct S
{
    string x;
    // NOTE: y is overloaded
    void y() {}
    ref y(return ref int z) { return z; }
    float y(float z) { return z; }

    // whether it returns rvalue or lvalue depends on what is passed
    auto ref tpl(Args...)(auto ref Args args) {
        static if (args.length)
            return args[0];
    }
}

static assert(__traits(hypotheticalIsLvalue, S.y)); // which `y` ???
static assert(__traits(hypotheticalIsLvalue, S.tpl)); // ???

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