According to my interpretation of §3.4.1/8 this code should compile. What am I missing?

问题

According to [basic.lookup.unqual]/8 from N4140 the following snippet should compile. But it doesn't in clang, gcc and vs2013.

struct C {
    void f(I) {}
    using I = int;
};

[basic.lookup.unqual]/8 (emphases are mine):

For the members of a class X, a name used in a member function body, in a default argument, in an exception-specification, in the brace-or-equal-initializer of a non-static data member (9.2), or in the definition of a class member outside of the definition of X, following the member’s declarator-id³¹, shall be declared in one of the following ways:

before its use in the block in which it is used or in an enclosing block (6.3), or

shall be a member of class X or be a member of a base class of X (10.2), or

...

^{31) That is, an unqualified name that occurs, for instance, in a type in the parameter-declaration-clause or in the
exceptions-pecification.}

回答1:

The way you are using it does not fall under any of the conditions in [basic.lookup.unqual]/8, it is not used:

in a member function body
in a default argument
in an exception-specification
in the brace-or-equal-initializer of a non-static data member

and it does not fall under this case either:

definition of a class member outside of the definition of X

the following example shows some cases that do follow those rules:

struct C {
    int y = I() ;           // brace-or-equal-initializer of non-static data member
    void f(int x = I())     // default argument
      noexcept(sizeof(I)<4) // exception-specification
    {
          I i = x ;         // member function body
    }
    using I = int;   
};

回答2:

[…] or in the definition of a class member outside of the definition of X, following the member’s declarator-id

I.e. the following is valid

struct C {
    using I = int;
    void f(I);
};

void C::f(I) {}

来源：https://stackoverflow.com/questions/31457627/according-to-my-interpretation-of-3-4-1-8-this-code-should-compile-what-am-i-m

标签

c++

language-lawyer

c++14