[Issue 19708] Can't use __traits(getAttributes, ...)[...] as a type

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19708

--- Comment #3 from Basile-z <b2.temp at gmx.com> ---
Most of the fix is easy but the part involving an index is more complicated
because there's a kind of precedence problem:

  alias d = __traits(getAttributes, bar)[0];

is more like (using parens to explain even if not valid)

  alias d = IndexedBy(__traits(getAttributes, bar), 0);

The internal wrapper is itself wrapped in the type that stores the index so a
flag indicating if symbols are allowed when processing the trait is never set.
That's why there's the spurious error message about giving a type.

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