Introspecting a range for .front `scope`ness

[Dukc]

I'm trying to improve `std.array : array` to infer whether it's 
parameter range can be `scope`. To do this, I need to introspect 
whether the `.front` of the range in question is `scope`. I also 
need to differentiate plain `scope` front from `scope return` or 
`return scope`. That is, I need to distinguish the first of these 
from the latter two:

```D
struct Range
{ // Can point to stack, won't return self-references.
   ElType front() scope;
   // Can point to stack, might return a self-reference or 
reference to pointed-to data.
   ElType front() scope return;
   // Can point to stack. If so, may return a pointer to stack, 
but not to this struct itself.
   ElType front() return scope;
}
```

I tried to solve it this way:

```D
static if(is(typeof((scope Range test, ElementType!Range[] out_) 
=> out_[0] = test.front)))
```

However, this passes even if `range.front` is `return scope` 
(which I want to fail this test). I suspect it's because scope 
checks are only done in `@safe` code. I can't test the assignment 
for safety since Phobos has to work with ranges that have 
`@system` `.front` or `.front.opAssign`.

I figured out I need to introspect the parameter storage class of 
`test.front`, but how? `std.traits : ParameterStorageClassTuple` 
would work, but it doesn't tell the storage classes of the hidden 
`.this` pointer - only those of the visible parameters.