https://issues.dlang.org/show_bug.cgi?id=19303

Luís Marques <luis@luismarques.eu> changed:

           What    |Removed                     |Added
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           Hardware|x86                         |All
                 OS|Mac OS X                    |All
           Severity|enhancement                 |normal

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https://issues.dlang.org/show_bug.cgi?id=19303

Eduard Staniloiu <edi33416@gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |edi33416@gmail.com

--- Comment #1 from Eduard Staniloiu <edi33416@gmail.com> ---
This is most definitely a compiler bug which seems to be related/triggered my the template mixin, as `std.traits.hasMember` only calls the equivalent `__traits(hasMember, T, memberName)` compiler trait.

As a workaround, you can use
static assert(__traits(hasMember, P, "ElementType")); // passes


Also, weirdly enough, swapping the mixin instantiation with the `static if` block in `Port`, will pass the initial assert

```
struct Port(SomeSignal)
{
    static if(hasMember!(SomeSignal, "ElementType"))
        alias ElementType = SomeSignal.ElementType;

    mixin SignalOps;
}

void main()
{
    ...
    static assert(hasMember!(P, "ElementType")); // OK
    static assert(__traits(hasMember, P, "ElementType")); // OK
}
```

You can check this on https://run.dlang.io/is/6aXcGq

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https://issues.dlang.org/show_bug.cgi?id=19303

--- Comment #2 from Eduard Staniloiu <edi33416@gmail.com> ---
What is even weirder, if you define your own `hasMember` wrapper, that works just fine with the initial definition of Port

```
enum hasM(T, string name) = __traits(hasMember, T, name);

static assert(hasM!(P, "ElementType")); // ok
```

As reflected by https://run.dlang.io/is/pKYL9B

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https://issues.dlang.org/show_bug.cgi?id=19303

Eduard Staniloiu <edi33416@gmail.com> changed:

           What    |Removed                     |Added
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          Component|phobos                      |dmd

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https://issues.dlang.org/show_bug.cgi?id=19303

Boris Carvajal <boris2.9@gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |boris2.9@gmail.com

--- Comment #3 from Boris Carvajal <boris2.9@gmail.com> ---
There are 2 issues here:

1) Currently, DMD can't see forward references that are declared inside subsequent declaration scopes (things like 'static if' or 'static foreach'), this is hard to solve because the compiler could jump to a later scope looking for what it needs but that scope could also need something from the previous, a chicken and egg problem.

2) Template instances are cached, if the first instance of 'hasMember!(S,
ElementType")' evaluates to false, it will always return false.

Code:
---
enum hasM(T, string name) = __traits(hasMember, T, name);

struct S
{
    // ElementType is not yet defined so it evaluates to false and the
    // pragma(hasM) below will print false.
    // Commenting out this 'static if' will make the pragma(hasM) print true
    // because the three pragmas are evaluated much later than 'struct S'.
    static if(hasM!(S, "ElementType")) { }

    static if(1)
        alias ElementType = int;
}

void main()
{
    pragma(msg, S.ElementType); // int
    pragma(msg, hasM!(S, "ElementType")); // depends on the first instantiation
    pragma(msg, __traits(hasMember, S, "ElementType"));  // true
}
---

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https://issues.dlang.org/show_bug.cgi?id=19303

Iain Buclaw <ibuclaw@gdcproject.org> changed:

           What    |Removed                     |Added
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           Priority|P1                          |P3

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