class 模板构造函数在c++20 中可以有多余的模板参数列表吗

Question

据我所知，以下code：

template<typename T>
struct S {
    S<T>();
};

格式正确，即使构造函数声明中的 <T> 是多余的。

然而，在 gcc trunk 上（但不是在 gcc10.2 上），使用 -std=c++20 这给出了一个错误：

error: expected unqualified-id before ')' token
    3 |     S<T>();
                 ^

code 使用 -std=c++20 在 clang trunk 上编译。这是一个错误，还是 c++20 中尚未在所有编译器中实现的重大更改？

Answer 1

事实上，发生了变化。它记录在 C++20 草案的兼容性部分。

[diff.cpp17.class]

2 Affected subclauses: [class.ctor] and [class.dtor]
Change: A simple-template-id is no longer valid as the declarator-id of a constructor or destructor.
Rationale: Remove potentially error-prone option for redundancy.
Effect on original feature: Valid C++ 2017 code may fail to compile in this International Standard. For example:
template<class T>
struct A {
  A<T>();           // error: simple-template-id not allowed for constructor
  A(int);           // OK, injected-class-name used
  ~A<T>();          // error: simple-template-id not allowed for destructor
};

具体来说，delta的写法是这样的：

n4659 - C++17 standard draft - [class.ctor]

1 Constructors do not have names. In a declaration of a constructor, the declarator is a function declarator of the form
ptr-declarator ( parameter-declaration-clause ) noexcept-specifier attribute-specifier-seq
where the ptr-declarator consists solely of an id-expression, an optional attribute-specifier-seq, and optional surrounding parentheses, and the id-expression has one of the following forms:

in a member-declaration that belongs to the member-specification of a class but is not a friend declaration, the id-expression is the injected-class-name of the immediately-enclosing class;

in a member-declaration that belongs to the member-specification of a class template but is not a friend declaration, the id-expression is a class-name that names the current instantiation of the immediately-enclosing class template; or

n4861 - C++20 standard draft - [class.ctor]

1 A constructor is introduced by a declaration whose declarator is a function declarator ([dcl.fct]) of the form
ptr-declarator ( parameter-declaration-clause ) noexcept-specifier attribute-specifier-seq
where the ptr-declarator consists solely of an id-expression, an optional attribute-specifier-seq, and optional surrounding parentheses, and the id-expression has one of the following forms:

in a member-declaration that belongs to the member-specification of a class or class template but is not a friend declaration ([class.friend]), the id-expression is the injected-class-name ([class.pre]) of the immediately-enclosing entity or

如您所见，措辞发生了变化。 C++20 现在在为 class 模板声明构造函数时需要注入的 class 名称。 S<T> 是一个简单的模板 ID，用于命名专业化。在模板中，注入的 class 名称只是 S.

这是地址 CWG 2237 的一部分。

class 模板构造函数在c++20 中可以有多余的模板参数列表吗

Can class template constructors have a redundant template parameter list in c++20

c++

templates

constructor

language-lawyer

c++20