[Issue 21807] Non-immutable data can be converted to immutable using function call in ctor

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21807

Paul Backus <snarwin+bugzilla at gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |snarwin+bugzilla at gmail.com

--- Comment #2 from Paul Backus <snarwin+bugzilla at gmail.com> ---
If `bb` and the constructor are marked as @safe, DMD produces the following
error:

---
Error: reference to stack allocated value returned by bb() assigned to
non-scope this.str
---

Separately from this, compiling with `-preview=dip1000` causes DMD to produce
the following error:

---
Error: cannot implicitly convert expression bb() of type char[12] to string
---

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