[Issue 24003] return/scope inference does not end up in type to some degree

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24003

Ate Eskola <Ajieskola at gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |Ajieskola at gmail.com

--- Comment #3 from Ate Eskola <Ajieskola at gmail.com> ---
I think this is same as what I was just about to report:

```d
auto nullPtr(int*) => (int*).init;

@safe unittest
{   int local;
    // compiles (with -preview=dip1000), because the parameter is correctly
inferred as scope
    auto nullP = nullPtr(&local);
}

// Yet this says AliasSeq!().
// Should say AliasSeq!"scope".
pragma(msg, __traits(getParameterStorageClasses, nullPtr, 0));
```

I was solving https://issues.dlang.org/show_bug.cgi?id=23300, but this bug is
blocking me. Since `scope` inference is limited, `std.array.array` needs to
manually introspect whether `.front` / `.opApply` / element copy of the source
range are `scope`. With this bug, it only works in the rare few cases where
`scope` is manually specified.

I have tried working around it by testing whether a call passing a `scope`
argument compiles, but no joy. DIP1000 checks are only done in safe code, and
trying to cast a `@system` function to a `@trusted` one to make the check work,
without changing the function type in any other way or losing the `scope`
inference, was just too difficult for me - probably impossible.

--