Nullable inout构造函数创建可变对象

Question

以下代码在试图编译时给出了一个奇怪的错误：

import std.conv: to;
import std.typecons;
import std.traits;

void main()
{
    alias BuiltinScalars = TypeTuple!(ubyte, byte, ushort, short, uint, int, ulong, long, 
                                      float, double, real, char, wchar, dchar, bool,
                                      ifloat, idouble, ireal, cfloat, cdouble, creal);

    foreach (T; BuiltinScalars)
    {
        foreach (ValT; BuiltinScalars)
        {
            alias KeyT = T;
            alias AAT = ValT[KeyT];
            foreach (NullableAAT; TypeTuple!(Nullable!AAT, const(Nullable!AAT), immutable(Nullable!AAT)))
            {
                NullableAAT naa;
                assert(naa.to!string() == "Nullable.null");

                static if (!isSomeString!KeyT)
                    enum KeyTInit = KeyT.init;
                else
                    enum KeyTInit = `""`;

                NullableAAT naav = [KeyTInit:ValT.init];
                assert(naav.to!string() == '[' ~ KeyTInit.to!string() ~ ':' ~ ValT.init.to!string() ~ ']');
            }
        }        
    }
}

我不知道这个代码有什么问题。Nullable只有一个构造函数，带有签名的this(inout T value) inout。

奇怪的事情(也许并不奇怪)。可能是编译器在出现了这么多错误之后就放弃了)，因为错误并不存在于所有类型的组合中，只有那些将ubyte作为一个关键类型的类型。完整的错误输出是：

bug.d(46): Error: inout constructor std.typecons.Nullable!(ubyte[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(byte[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(ushort[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(short[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(uint[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(int[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(ulong[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(long[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(float[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(double[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(real[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(char[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(wchar[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(dchar[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(bool[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(ifloat[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(idouble[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(ireal[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(cfloat[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(cdouble[ubyte]).Nullable.this creates mutable object, not immutable
bug.d(46): Error: inout constructor std.typecons.Nullable!(creal[ubyte]).Nullable.this creates mutable object, not immutable

Answer 1

默认情况下，关联数组文字是可变的。

要修复，您可以替换：

            NullableAAT naav = [KeyTInit:ValT.init];

通过以下方式：

            alias CAA = typeof(naa.get());
            CAA aa = [KeyTInit:ValT.init];
            NullableAAT naav = aa;

这将以正确的常数声明AA，这将通过构造函数的Nullable传播到inout。

奇怪的事情(也许并不奇怪)。可能是编译器在出现了这么多错误之后就放弃了)，因为错误并不存在于所有类型的组合中，只有那些将ubyte作为一个关键类型的类型。

是的，它在第二个循环的第一个迭代之后停止。您可以通过从元组的头部移除类型来进行测试。