c++ - 结构化绑定(bind)初始化程序表单 { assignment-expression } 在 clang 上的数组类型失败

Question

[dcl.dcl]/1 (最终 C++17 草案，N4659)将简单声明的语法描述为:

[...]

simple-declaration: - [...] - attribute-specifier-seq(opt) decl-specifier-seq ref-qualifier(opt) [ identifier-list ] initializer;

[dcl.dcl]/8描述了简单声明的后一种形式是结构化绑定(bind)声明:

A simple-declaration with an identifier-list is called a structured binding declaration ([dcl.struct.bind]). The decl-specifier-seq shall contain only the type-specifier auto and cv-qualifier:s. The initializer shall be of the form “= assignment-expression”, of the form “{ assignment-expression }”, or of the form “( assignment-expression )”, where the assignment-expression is of array or non-union class type.

即，就这个问题而言，结构化绑定(bind)具有简化的语法:

auto [ identifier-list ] initializer ;

以下任何形式都是有效的初始化程序:s:

... = assignment-expression

... { assignment-expression }

... ( assignment-expression )

因此，可以说以下代码格式正确:

struct S { int s; };

int main() {
    const S s{42};
    const int arr[1] = {42};

    // ... of the form “= assignment-expression”
    auto[s_form1] = s;
    auto[e_form1] = arr;

    // ... of the form “{ assignment-expression }”
    auto[s_form2]{s};
    auto[e_form2]{arr};

    // ... of the form “( assignment-expression )”
    auto[s_form3](s);
    auto[e_form3](arr);

    (void)s_form1; (void)s_form2; (void)s_form3;
    (void)e_form1; (void)e_form2; (void)e_form3;

    return 0;
}

同时使用 -std=c++17和 -std=c++2a , GCC (9.3) 接受此代码，而 clang (10.0.0 以及 HEAD/11) 拒绝数组的“{ assignment-expression }”形式:

auto[e_form2]{arr};
              ^~~
error: cannot initialize an array element of type 'const int'
       with an lvalue of type 'const int [1]'

对于右值数组，它同样失败:

using SingleElementIntArray = int[1];
auto[e_form2]{SingleElementIntArray{42}};
              ^~~~~~~~~~~~~~~~~~~~~~~~~
error: cannot initialize an array element of type
       'int' with an rvalue of type 
       'SingleElementIntArray' (aka 'int [1]')

问题

谁在这里，GCC 还是 clang？我的猜测是 GCC；如果是这样，这是一个已知的clang错误吗？

Answer 1

这是一个已知的错误，有一个开放的错误报告

LLVM 打开错误报告:

Bug 32466 - Clang doesn't support structured binding of initializer form {" expr }" for array

Ryou Ezoe 2017-03-30 00:24:07 PDT:

Clang doesn't support structured binding of initializer form {" expr }" if expr is id-expression of array object. Clang does not accept following code.

int expr[] = { 1,2,3 } ;
auto [a,b,c]{expr} ;

with the error:

prog.cc:4:18: error: cannot initialize an array element of type 'int' with an lvalue of type 'int [3]'

其他情况按预期工作。 "( expr )"和 "{
expr }"其中 expr 是类对象的 id 表达式:

auto [a,b,c](expr) ;

struct X { int x,y,z } ; 
X x {1,2,3} ; 
auto [a,b,c]{ x } ;