C# 编译器错误：允许从 Nullable<decimal> 转换为十进制

Question

考虑以下代码：

    public class DecimalWrapper
    {
        public static implicit operator DecimalWrapper(decimal x) => new();
    }

    [Fact]
    public void Test()
    {
        // Why this even compiles? Since there is no implicit conversion from decimal? -> decimal
        DecimalWrapper c = (decimal?)null; // variable 'c' is null!
    }

我根本不希望它编译，因为没有十进制的隐式转换？到十进制。

我认为这是一个错误还是我弄错了什么？

我看过这个： Serious bugs with lifted/nullable conversions from int, allowing conversion from decimal

但这看起来不一样，而且已经过时（7 年以上），所以这些错误现在应该已修复，但不能确定，因为所有指向错误报告的链接都消失了）...:(

我真的很想在实际解决方案（计算跟踪）中使用这样的代码，但这阻止了我。

PS：我正在 Windows.

上编译

Answer 1

根据specification lifted conversion operators are allowed only for value type to value type conversions (and their nullable counterparts) but in the Roslyn compiler source code你可以找到下一条评论：

DELIBERATE SPEC VIOLATION:
The native compiler allows for a "lifted" conversion even when the return type of the conversion not a non-nullable value type. For example, if we have a conversion from struct S to string, then a "lifted" conversion from S? to string is considered by the native compiler to exist, with the semantics of "s.HasValue ? (string)s.Value : (string)null". The Roslyn compiler perpetuates this error for the sake of backwards compatibility.

所以这似乎是为了向后兼容而引入的实际错误。而 decompilation 恰好显示了这种行为。