opAssign is not callable because it is annotated with @disable with SysTime(Possible bug?)

[Jean-Mathieu Deschenes via Digitalmars-d-learn]

Hello,

I am currently trying to improve the TOML library of: 
https://github.com/iccodegr/toml.d

I get a really weird error:

Error: function main.TOMLValue.opAssign is not callable because 
it is annotated with @disable

I have tested this with dmd 2.071.1 and 2.068.0.

Here is the code to reproduce it:

import std.datetime: SysTime, DateTime;
import std.stdio;
import std.conv;
import std.exception;

enum TOMLType {
    String,
    Integer,
    Float,
    Boolean,
    DateTime,
    Array,
    Group
}

// Changing this to DateTime makes it compile
alias TOMLDateTimeType = SysTime;

class TOMLException: Exception {
    this(string msg, string file="parser", size_t line=111) {
        super(msg, file, line);
    }
}

alias enforceTOML = enforceEx!(TOMLException);


struct TOMLValue {

    union Store {
        string stringv;
        long intv;
        float floatv;
        bool boolv;
        TOMLDateTimeType datetimev;
        TOMLValue[] arrayv;
        TOMLValue[string] keygroups;
    }

    private {
        Store _store;
        TOMLType _type;
    }

    // Does not work with this either

    //private void assign(T)(T val, string key) {
    //    static if ( is(T: TOMLValue) )
    //        _store.keygroups[key] = val;
    //    else
    //        _store.keygroups[key] = TOMLValue(val);
    //}

    TOMLValue opIndexAssign(TOMLValue  v, string key) {
        enforceTOML(_type==TOMLType.Group);
        _store.keygroups[key] = v;
        return v;
    }


}


void main() {
        TOMLType a = TOMLType.String;
        writeln(a.to!string);


}

It doesn't really make any sense to me. Could anyone explain to 
me what's going on?

Thank you for your time