[Issue 19786] alias to __traits(getMember) wrongfully always binds to this

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19786

--- Comment #4 from Basile-z <b2.temp at gmx.com> ---
I wont fix the issue since it's assigned but just a few words to clarify the
problem:

1. The wrong symbol is return because of a call to `getDsymbolWithoutExp()`
(https://github.com/dlang/dmd/blob/93b4b99b0b024d238b223e73804c1d0c01ee42ec/src/dmd/typesem.d#L1637)

2. After this call, a type is returned from the `TraitExp`, always, even when
the `TraitsExp` doesn't represent a `Type`. That's because `getType()` returns
the type of the expression when the expression itself doesn't represents/wraps
a `Type`.

The fix would consist into detecting the invalid stuff beforehand

---
if (Expression e = semanticTraits(mtype.exp, sc))
{
    if (TupleExp te = e.toTupleExp)
        mtype.sym = new TupleDeclaration(mtype.loc,
            Identifier.generateId("__aliastup"), cast(Objects*) te.exps);
    else if (Dsymbol ds = getDsymbol(e))
        mtype.sym = ds;

    // detect invalid cases such as 19786 here at worst

    // if you reach this point with smthg like issue 19786 it's too late
    else if (Dsymbol ds = getDsymbolWithoutExpCtx(e))
        mtype.sym = ds;

    // if you reach this point with an invalid case it's too late
    else if (Type t = getType(e))
        result = t.addMod(mtype.mod);
}
---

Note in reaction to previous comment, `rhs` is not an expression, it's a
`VarDeclaration`, so a symbol.

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