c++ - c++17 中的非类型模板参数可以是 decltype(auto) 吗？

Question

我发现 gcc 和 clang 允许在非类型模板参数类型子句中使用 decltype(auto)。例如:

template <decltype(auto)>
struct X {};

int foo ;

int main() {
    X<(foo)> x;
    static_cast<void>(x);
}

[live demo gcc] [live demo clang]

它是符合标准的功能还是某种 gnu 扩展？

Answer 1

这是标准的。首先，对于非类型模板参数:

[temp.param/4]

A non-type template-parameter shall have one of the following (optionally cv-qualified) types:

• ...
• a type that contains a placeholder type.

其中占位符类型指定了以下内容:

[dcl.spec.auto/1]

The auto and decltype(auto) type-specifiers are used to designate a placeholder type that will be replaced later by deduction from an initializer. The auto type-specifier is also used to introduce a function type having a trailing-return-type or to signify that a lambda is a generic lambda ([expr.prim.lambda.closure]). The auto type-specifier is also used to introduce a structured binding declaration.

[dcl.spec.auto/5]

A placeholder type can also be used in the type-specifier-seq in the new-type-id or type-id of a new-expression and as a decl-specifier of the parameter-declaration's decl-specifier-seq in a template-parameter.

由于上面的项目符号说的是“占位符类型”，并且这种类型可以用 auto 或 decltype(auto) 指定，所以两种编译器都是正确的。