[Issue 22572] Cannot define SumType over immutable struct with Nullable

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22572

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

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

--- Comment #1 from Paul Backus <snarwin+bugzilla at gmail.com> ---
There's definitely a SumType issue here:

---
import std.sumtype;

struct CopyConstruct
{
    this(ref inout CopyConstruct other) inout { }
}

immutable struct Value
{
    CopyConstruct c;
}

alias _ = SumType!Value;
---

The example above produces the following error message, as of DMD/Phobos
2.098.1:

---
/dlang/dmd/linux/bin64/../../src/phobos/std/sumtype.d(1998): Error: static
assert:  "`handlers[0]` of type `template` never matches"
/dlang/dmd/linux/bin64/../../src/phobos/std/sumtype.d(1590):       
instantiated from here: `matchImpl!(inout(SumType!(immutable(Value))))`
/dlang/dmd/linux/bin64/../../src/phobos/std/sumtype.d(452):        instantiated
from here: `match!(inout(SumType!(immutable(Value))))`
onlineapp.d(13):        instantiated from here: `SumType!(immutable(Value))`
---

The source of the error is this line:

https://github.com/dlang/phobos/blob/v2.098.1/std/sumtype.d#L452

Compiling with -verrors=spec reveals the following gagged error, which is
ultimately responsible for the failure:

---
(spec:1) src/sumtype.d(389): Error: `inout` on `return` means `inout` must be
on a parameter as well for `pure nothrow @nogc @safe inout(Storage)(ref
immutable(Value) value)`
---

SumType's copy constructor incorrectly assumes that, when copying from an
`inout(SumType) other`, the value stored in `other` will also be
inout-qualified. However, this is not the case when one of other's members is
an immutable struct, because the qualifier combination `immutable inout`
collapses to just `immutable`.

--