Why does Clang++ compiler fail to compile the following variadic template code?

Question

#include <string>
#include <iostream>
#include <tuple>
#include <utility>


template<typename... T> struct test {

    using args_type = std::tuple<T...>;

    args_type x;

    template<std::size_t... I>
    void callme(std::index_sequence<I...>) {
        int _[] = {(std::get<I>(x).std::tuple_element<I, args_type>::type::~type(), true)...};
    }
};

int main() {
}

The error message is

clang-3.7  -std=gnu++1y  -Wc++14-extensions test.cpp
test.cpp:15:56: error: expected ')'
        int _[] = {(std::get<I>(x).std::tuple_element<I, args_type>::type::~type(), true)...};
                                                       ^
test.cpp:15:20: note: to match this '('
        int _[] = {(std::get<I>(x).std::tuple_element<I, args_type>::type::~type(), true)...};
                   ^
1 error generated.

The same code seems to compile just fine with G++ 4.9.2. I couldn't find any relevant bug report on Clang yet.

Where did this copy of clang 3.7 come from? The most recent release of LLVM is only 3.6. — Bill Lynch
– Bill Lynch, Commented May 12, 2015 at 21:11
.std::tuple_element<I, args_type>::type::~type() is supposed to call the destructor of the element? How is ::type not a dependent type there? — Yakk - Adam Nevraumont
– Yakk - Adam Nevraumont, Commented May 13, 2015 at 18:33

Columbo · Accepted Answer · 2015-05-12 21:40:52Z

10

Appears to be a Clang bug, though the lookup of such pseudo-destructor-names is probably defected and subject of open CWG issues, specifically 555 and 399.

The significant bit of the expansion pattern is

std::get<I>(x).std::tuple_element<I, args_type>::type::~type()

Here, the bit between . and () is a pseudo-destructor-name; Qualified name lookup then mandates that

If a pseudo-destructor-name (5.2.4) contains a nested-name-specifier, the type-names are looked up as types in the scope designated by the nested-name-specifier. Similarly, in a qualified-id of the form:

nested-name-specifier_opt class-name :: ~ class-name

the second class-name is looked up in the same scope as the first.

I.e. type is looked up in std::tuple_element<I, args_type>, where it's found to refer to some type. Note that class-name is a grammatical name for identifiers (and simple-template-ids), and need not refer to an actual class. std::get<I>(x).std::tuple_element<I, args_type>::type::~type then refers to the destructor of type.

Workaround with an auxiliary function:

template <typename T>
void destroy(T& p) {p.~T();}

template<typename... T> struct test {
    using args_type = std::tuple<T...>;

    args_type x;

    template<std::size_t... I>
    void callme(std::index_sequence<I...>) {
        int _[] = {(destroy(std::get<I>(x)), 0)...};
    }
};

edited May 12, 2015 at 21:40

answered May 12, 2015 at 21:16

Columbo

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7 Comments

Qix - MONICA WAS MISTREATED Over a year ago

I would file a report. That's odd.

T.C. Over a year ago

The specification for destructor name lookup is all sorts of messed up. There's at least one longstanding core issue about it.

Columbo Over a year ago

@T.C. I just realized how weird this really is.

T.C. Over a year ago

@Columbo 399, too. Also, this one is not necessarily a pseudo-destructor. Could be a real one.

Nat Over a year ago

Is the code actually complied with C++14 specification though i.e. it's actually GCC bug? (open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf section 5.2.4 page 100)

|

Nat · Accepted Answer · 2015-05-19 04:25:31Z

0

Interestingly, hstong from IBM mentioned a couple workarounds that work better.

int _[] = {(std::get<I>(x).::std::tuple_element<I, args_type>::type::~type(), true)...};

or

int _[] = {(std::get<I>(x).std::template tuple_element<I, args_type>::type::~type(), true)...};

answered May 19, 2015 at 4:25

Nat

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