c++ - 在类内部，为什么 `auto b() -> decltype(a()) {}` 有效，但 `decltype(a()) b() {}` 无效？

Question

考虑以下代码:(Ideone)

struct S
{
    int a() {return 0;}
    decltype(a()) b() {return 1;}
};

它给了我以下错误:

error: cannot call member function 'int S::a()' without object

另一方面，这段代码编译得很好:(Ideone)

struct S
{
    int a() {return 0;}
    auto b() -> decltype(a()) {return 1;}
};

为什么一个例子有效，而另一个却编译失败？

两个示例中的编译器行为是否完全正确？

如果编译器是正确的，那么为什么标准会要求这种奇怪的行为？

Answer 1

由于a是一个非静态成员函数，a()被解释为(*this).a()。部分引自 [expr.prim.general]/3，

If a declaration declares a member function or member function template of a class X, the expression this is a prvalue of type “pointer to cv-qualifier-seq X” between the optional cv-qualifer-seq and the end of the function-definition, member-declarator, or declarator. It shall not appear before the optional cv-qualifier-seq and it shall not appear within the declaration of a static member function (although its type and value category are defined within a static member function as they are within a non-static member function).

trailing-return-type 出现在可选的 cv-qualifier-seq 之后(在您的示例中省略，因为 S::b不是 cv 限定的)所以 this 可以出现在那里，但它不能出现在之前。