[Issue 20175] New: Nullable!T.get(x) immutable array elements downgraded to const

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20175

          Issue ID: 20175
           Summary: Nullable!T.get(x) immutable array elements downgraded
                    to const
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: elpenguino+D at gmail.com

```
import std.typecons : Nullable;
Nullable!string x;
static assert(is(typeof(x.get()) == string)); //OK!
static assert(is(typeof(x.get("")) == string)); //UH OH
```

We get const(char)[] instead of the expected string.

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