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From: "bruno-gcc at defraine dot net" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/54359] New: [C++0x][N3282] decltype in member function's trailing return type when defined outside of class
Date: Thu, 23 Aug 2012 13:42:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54359

             Bug #: 54359
           Summary: [C++0x][N3282] decltype in member function's trailing
                    return type when defined outside of class
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: bruno-gcc@defraine.net


Even after closing bug #49003, there remain problems regarding the handling of
N3282 (resolution of DR 1207). Specifically, the problems have to do with a
definition of a member function outside of the class.

Consider these test cases:

int& ref(int& x) { return x; }
const int& ref(const int& x) { return x; }

class A {
    int x;
    int f() const;
    auto test1() const -> decltype(this);
    auto test2() const -> decltype(ref(x));
    auto test3() const -> decltype(f());
};

auto A::test1() const -> decltype(this) {
    return this;
}

auto A::test2() const -> decltype(ref(x)) {
    return ref(x);
}

auto A::test3() const -> decltype(f()) {
    return f();
}

This gives the following compiler errors in the C++0x mode of gcc 4.7.1:

- test1(): invalid use of 'this' at top level
- test2(): prototype for 'int& A::test2() const' does not match any in class
'A' (candidate is: const int& A::test2() const); additionally, 'int A::x' is
private
- test3(): cannot call member function 'int A::f() const' without object;
additionally, 'int A::f() const' is private

In summary, in the trailing return type of a const member function definition
outside of a class declaration, it seems:
1. I cannot explicitly reference 'this'
2. I can reference data members, but not private ones, and they are not const
3. I cannot invoke member functions

I believe all three function definitions should compile. Similarly, inline
definitions of the same member functions have no such problems:

class B {
    int x;
    int f() const;
    auto test1() const -> decltype(this) { return this; }
    auto test2() const -> decltype(ref(x)) { return ref(x); }
    auto test3() const -> decltype(f()) { return f(); }
};

It seems that issue #2 is a regression compared to gcc 4.6, which seemed to
allow some access to data members. Consider test4():

template<typename X>
class C {
    const X& x;
    auto test4() const -> decltype(x);
};

template<typename X>
auto C<X>::test4() const -> decltype(x) {
    return x;
}

This test compiles without problems under gcc 4.6.3. Whereas gcc 4.7.1 gives:

prototype for 'decltype (((C<X>*)0)->C<X>::x) C<X>::test4() const' does not
match any in class 'C<X>' (candidate is: decltype (((const
C<X>*)this)->C<X>::x) C<X>::test4() const)