為什麼編譯器不能「合成「純虛析構函數的函數定義？？

雖然編譯器採取」分離編譯模型「，但是編譯器完全可以在編譯包含純虛析構函數的class時」合成「出純虛析構函數的函數定義體，不是嗎？

An abstract class is a class that can be used only as a base class of some other class; no objects of an abstract class can be created except as subobjects of a class derived from it. A class is abstract if it has at least one pure virtual function. A virtual function is specified pure by using a pure-specifier in the function declaration in the class definition. A pure virtual function need be defined only if called with, or as if with, the qualified-id syntax.

C++標準§ 12.4第4條：

If a class has no user-declared destructor, a destructor is implicitly declared as defaulted. An implicitlydeclared destructor is an inline public member of its class.

如果想讓一個函數成為純虛函數，必須得聲明它。而一旦聲明了析構函數，不管它是不是（純）虛函數，你都必須要定義它。

class Base
{
public:
virtual ~Base() = 0;
}

Base::~Base() = default;