• via Digitalmars-d-bugs (66/67) Jun 25 2017 https://issues.dlang.org/show_bug.cgi?id=17552

via Digitalmars-d-bugs <digitalmars-d-bugs puremagic.com> writes:

https://issues.dlang.org/show_bug.cgi?id=17552

          Issue ID: 17552
           Summary: Cannot implicitly convert expression (struct this)
           Product: D
           Version: D2
          Hardware: x86
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody puremagic.com
          Reporter: andre s-e-a-p.de

I created a custom type which enables me to have enums which have as their
initial state, the init value of their base type. Something similiar to
Nullable...

enum Reason : string {CO = "Co", FU = "Fu", CA = "Ca"}
struct TestStruct {InitialEnum!Reason reason;}

This line raises the error:
TestStruct s2 = TestStruct(Reason.FU);
 Error: cannot implicitly convert expression ("Fu") of type Reason to
InitialEnum!(Reason)


While this line is working fine:
TestStruct s1 = {reason: Reason.FU};

I asked this question in the learning forum and got the advice to create an
issue.

struct InitialEnum(T)
{
    import std.traits: OriginalType, EnumMembers;
    import std.conv: to;

    private T _value;
    private bool _isEmpty = true;
    alias EnumBaseType = OriginalType!T;

     property EnumBaseType baseTypeValue() { return (_isEmpty) ?
EnumBaseType.init : _value; }
     property T value()    { return _value; }
     property bool isEmpty() { return _isEmpty;    }

    alias baseTypeValue this;

    void opAssign(EnumBaseType value)
    {
        if (value == EnumBaseType.init)
        {
            _isEmpty = true;
            return;
        }

        foreach (member; EnumMembers!T)
        {
            if (value == member)
            {
                _value = member;
                _isEmpty = false;
                return;
            }        
        }
        throw new Exception("Value "~value.to!string~" is not a valid enum
value");
    }

    this(T t)
    {
        _isEmpty = false;
        _value = t;
    }

    this(EnumBaseType value)
    {
        opAssign(value);
    }
}

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