[Bug c++/54738] New: [C++11][SFINAE] Hard errors for pointer-to-member function expressions

["daniel.kruegler at googlemail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54738

             Bug #: 54738
           Summary: [C++11][SFINAE] Hard errors for pointer-to-member
                    function expressions
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: daniel.kruegler@googlemail.com


During my attempts to implement sfinae-friendly result_of I found the following
problems using gcc 4.8.0 20120923 (experimental) with compile-flags

-Wall -pedantic -std=c++0x

with pointer-to-member function expressions. 

First there are problems if we have an incompatible number of function call
arguments:

//---
template<class T>
T&& declval();

template<class F, class T1, class... Ts>
decltype(((*declval<T1>()).*declval<F>())(declval<Ts>()...))
test1(int);

template<class...>
void test1(...);

template<class F, class T1, class... Ts>
decltype((declval<T1>().*declval<F>())(declval<Ts>()...))
test2(int);

template<class...>
void test2(...);

struct S {};

typedef void (S::*Func)(int) const;

typedef decltype(test1<Func, S*>(0)) type1a;  // #22
typedef decltype(test1<Func, S*&>(0)) type1b;
typedef decltype(test1<Func, S*, int, int>(0)) type1c;
typedef decltype(test1<Func, S*&, int, int>(0)) type1d;

typedef decltype(test2<Func, S>(0)) type2a; // #27
typedef decltype(test2<Func, S&>(0)) type2b;
typedef decltype(test2<Func, S, int, int>(0)) type2c;
typedef decltype(test2<Func, S&, int, int>(0)) type2d;
//---

The error messages are:

<quote>
22|  required from here|
6|error: too few arguments to function|
22|  required from here|
6|error: too few arguments to function|
23|  required from here|
6|error: too few arguments to function|
23|  required from here|
6|error: too few arguments to function|
24|  required from here|
6|error: too many arguments to function|
25|  required from here|
6|error: too many arguments to function|
27|  required from here|
13|error: too few arguments to function|
27|  required from here|
13|error: too few arguments to function|
28|  required from here|
13|error: too few arguments to function|
28|  required from here|
13|error: too few arguments to function|
29|  required from here|
13|error: too many arguments to function|
30|  required from here|
13|error: too many arguments to function|
</quote>

Further there are also problems with corresponding function call expressions
where the arguments won't convert:

//---
template<class T>
T&& declval();

template<class F, class T1, class... Ts>
decltype(((*declval<T1>()).*declval<F>())(declval<Ts>()...))
test1(int);

template<class...>
void test1(...);

template<class F, class T1, class... Ts>
decltype((declval<T1>().*declval<F>())(declval<Ts>()...))
test2(int);

template<class...>
void test2(...);

struct S {};

typedef void (S::*Func)(int) const;

typedef decltype(test1<Func, S*, S>(0)) type3a; // #22
typedef decltype(test1<Func, S*&, S>(0)) type3b;

typedef decltype(test2<Func, S, S>(0)) type4a; // #25
typedef decltype(test2<Func, S&, S>(0)) type4b;
//---

The error messages are:

<quote>
22|  required from here|
6|error: cannot convert 'S' to 'int' in argument passing|
22|  required from here|
6|error: cannot convert 'S' to 'int' in argument passing|
23|  required from here|
6|error: cannot convert 'S' to 'int' in argument passing|
23|  required from here|
6|error: cannot convert 'S' to 'int' in argument passing|
25|  required from here|
13|error: cannot convert 'S' to 'int' in argument passing|
25|  required from here|
13|error: cannot convert 'S' to 'int' in argument passing|
26|  required from here|
13|error: cannot convert 'S' to 'int' in argument passing|
26|  required from here|
13|error: cannot convert 'S' to 'int' in argument passing|
</quote>

Further cv-violations aren't sfinaed away:

//---
template<class T>
T&& declval();

template<class F, class T1, class... Ts>
decltype(((*declval<T1>()).*declval<F>())(declval<Ts>()...))
test1(int);

template<class...>
void test1(...);

template<class F, class T1, class... Ts>
decltype((declval<T1>().*declval<F>())(declval<Ts>()...))
test2(int);

template<class...>
void test2(...);

struct S {};

typedef void (S::*Func2)(int);

typedef decltype(test1<Func2, const S*, int>(0)) type5a; // #22
typedef decltype(test1<Func2, const S*&, int>(0)) type5b;

typedef decltype(test2<Func2, const S, int>(0)) type6a; // #25
typedef decltype(test2<Func2, const S&, int>(0)) type6b;
//---

The error messages are:

<quote>
22|  required from here|
6|error: invalid conversion from 'const S*' to 'S*' [-fpermissive]|
23|  required from here|
6|error: invalid conversion from 'const S*' to 'S*' [-fpermissive]|
25|  required from here|
13|error: invalid conversion from 'const S*' to 'S*' [-fpermissive]|
26|  required from here|
13|error: invalid conversion from 'const S*' to 'S*' [-fpermissive]|
</quote>

These code examples should all be accepted.