[Issue 23844] New: chain(only) doesn't support immutable structs

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23844

          Issue ID: 23844
           Summary: chain(only) doesn't support immutable structs
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: minor
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: default_357-line at yahoo.de

Consider this code:

```
void main() {
    import std.array : array;
    import std.range : chain, only;

    struct S
    {
        immutable int value;
    }

    auto range = chain(only(S(5)), only(S(6)));
    assert(range.array == [S(5), S(6)]);
}
```

It errors with a flood of 

```
std/range/package.d(1172): Error: cannot modify struct instance
`result.__source_field_0` of type `OnlyResult!(S)` because it contains `const`
or `immutable` members
```

If I remove `immutable`, it works.

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