P1359R0 [basic.lval] 措辞变化和别名规则
P1359R0 [basic.lval] wording change and aliasing rules
C++ 标准的 N4778 草案 (2018) 包含以下部分：
7.2.1 [basic.lval]
If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined:
- the dynamic type of the object,
- a cv-qualified version of the dynamic type of the object,
- a type similar (as defined in 7.5) to the dynamic type of the object,
- a type that is the signed or unsigned type corresponding to the dynamic type of the object,
- a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic typeof the object,
- an aggregate or union type that includes one of the aforementioned types among its elements or non-static data members (including, recursively, an element or non-static data member of a subaggregate or contained union),
- a type that is a (possibly cv-qualified) base class type of the dynamic type of the object,
- a
char, unsigned char, or std::byte type.
然而，draft at eel.is/c++draft/ lists只有3颗子弹（我突出了常见的）：
If a program attempts to access the stored value of an object through a glvalue whose type is not similar ([conv.qual]) to one of the following types the behavior is undefined:
- the dynamic type of the object,
- a type that is the signed or unsigned type corresponding to the dynamic type of the object,
- a
char, unsigned char, or std::byte type.
P1359R0 解释变化：
The aliasing rules of [basic.lval] were adapted from C with additions for C++. However, a number of the points either do not apply or are subsumed by other points. For example, the provision for aggregate and union types is needed in C for struct assignment, which in C++ is done via constructors and assignment operators in C++, not by accessing the complete object. ... It has been suggested that the aliasing rules should be extended to permit an object of an enumeration with a fixed underlying type to alias an object with that underlying type.
能否通俗地解释一下这种措辞变化的实际后果？据我了解，建议 enum class E : T { ... }; 允许别名 T。这个对吗？除了这个还有什么会变成defined/undefined吗？
没有实际后果。
这是一项内务清理，旨在删除 redundant/unused 从 C（或旧版 C++ 标准）继承的规则。它不太符合 "editorial" 的标准，因为它不仅仅是修补 grammar/spelling/formatting，但它也不是语义更改。
您提到的 enum 相关更改并未发生；这只是一个建议，在问题描述中的 "additional note" 中提到。我们可以看到这种情况在未来发生。我个人认为这是个好主意。
C++ 标准的 N4778 草案 (2018) 包含以下部分：
7.2.1 [basic.lval]
If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined:
- the dynamic type of the object,
- a cv-qualified version of the dynamic type of the object,
- a type similar (as defined in 7.5) to the dynamic type of the object,
- a type that is the signed or unsigned type corresponding to the dynamic type of the object,
- a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic typeof the object,
- an aggregate or union type that includes one of the aforementioned types among its elements or non-static data members (including, recursively, an element or non-static data member of a subaggregate or contained union),
- a type that is a (possibly cv-qualified) base class type of the dynamic type of the object,
- a
char,unsigned char, orstd::bytetype.
然而，draft at eel.is/c++draft/ lists只有3颗子弹（我突出了常见的）：
If a program attempts to access the stored value of an object through a glvalue whose type is not similar ([conv.qual]) to one of the following types the behavior is undefined:
- the dynamic type of the object,
- a type that is the signed or unsigned type corresponding to the dynamic type of the object,
- a
char,unsigned char, orstd::bytetype.
P1359R0 解释变化：
The aliasing rules of [basic.lval] were adapted from C with additions for C++. However, a number of the points either do not apply or are subsumed by other points. For example, the provision for aggregate and union types is needed in C for struct assignment, which in C++ is done via constructors and assignment operators in C++, not by accessing the complete object. ... It has been suggested that the aliasing rules should be extended to permit an object of an enumeration with a fixed underlying type to alias an object with that underlying type.
能否通俗地解释一下这种措辞变化的实际后果？据我了解，建议 enum class E : T { ... }; 允许别名 T。这个对吗？除了这个还有什么会变成defined/undefined吗？
没有实际后果。
这是一项内务清理，旨在删除 redundant/unused 从 C（或旧版 C++ 标准）继承的规则。它不太符合 "editorial" 的标准，因为它不仅仅是修补 grammar/spelling/formatting，但它也不是语义更改。
您提到的 enum 相关更改并未发生；这只是一个建议，在问题描述中的 "additional note" 中提到。我们可以看到这种情况在未来发生。我个人认为这是个好主意。