da1818a08c
Summary:
Currently parts of the SFINAE on tuples default constructor always gets evaluated even when the default constructor is never called or instantiated. This can cause a hard compile error when a tuple is created with types that do not have a default constructor. Below is a self contained example using a pair like class. This code will not compile but probably should.
```
#include <type_traits>
template <class T>
struct IllFormedDefaultImp {
IllFormedDefaultImp(T x) : value(x) {}
constexpr IllFormedDefaultImp() {}
T value;
};
typedef IllFormedDefaultImp<int &> IllFormedDefault;
template <class T, class U>
struct pair
{
template <bool Dummy = true,
class = typename std::enable_if<
std::is_default_constructible<T>::value
&& std::is_default_constructible<U>::value
&& Dummy>::type
>
constexpr pair() : first(), second() {}
pair(T const & t, U const & u) : first(t), second(u) {}
T first;
U second;
};
int main()
{
int x = 1;
IllFormedDefault v(x);
pair<IllFormedDefault, IllFormedDefault> p(v, v);
}
```
One way to fix this is to use `Dummy` in a more involved way in the constructor SFINAE. The following patch fixes these sorts of hard compile errors for tuple.
Reviewers: mclow.lists, rsmith, K-ballo, EricWF
Reviewed By: EricWF
Subscribers: ldionne, cfe-commits
Differential Revision: http://reviews.llvm.org/D7569
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@230120 91177308-0d34-0410-b5e6-96231b3b80d8
111 lines
3.0 KiB
C++
111 lines
3.0 KiB
C++
//===----------------------------------------------------------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is dual licensed under the MIT and the University of Illinois Open
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// Source Licenses. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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// <tuple>
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// template <class... Types> class tuple;
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// constexpr tuple();
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// UNSUPPORTED: c++98, c++03
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#include <tuple>
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#include <string>
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#include <cassert>
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#include <type_traits>
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#include "DefaultOnly.h"
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struct NoDefault {
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NoDefault() = delete;
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explicit NoDefault(int) { }
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};
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struct NoExceptDefault {
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NoExceptDefault() noexcept = default;
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};
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struct ThrowingDefault {
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ThrowingDefault() { }
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};
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struct IllFormedDefault {
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IllFormedDefault(int x) : value(x) {}
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template <bool Pred = false>
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constexpr IllFormedDefault() {
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static_assert(Pred,
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"The default constructor should not be instantiated");
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}
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int value;
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};
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int main()
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{
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{
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std::tuple<> t;
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}
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{
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std::tuple<int> t;
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assert(std::get<0>(t) == 0);
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}
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{
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std::tuple<int, char*> t;
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assert(std::get<0>(t) == 0);
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assert(std::get<1>(t) == nullptr);
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}
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{
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std::tuple<int, char*, std::string> t;
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assert(std::get<0>(t) == 0);
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assert(std::get<1>(t) == nullptr);
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assert(std::get<2>(t) == "");
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}
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{
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std::tuple<int, char*, std::string, DefaultOnly> t;
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assert(std::get<0>(t) == 0);
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assert(std::get<1>(t) == nullptr);
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assert(std::get<2>(t) == "");
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assert(std::get<3>(t) == DefaultOnly());
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}
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{
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// See bug #21157.
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static_assert(!std::is_default_constructible<std::tuple<NoDefault>>(), "");
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static_assert(!std::is_default_constructible<std::tuple<DefaultOnly, NoDefault>>(), "");
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static_assert(!std::is_default_constructible<std::tuple<NoDefault, DefaultOnly, NoDefault>>(), "");
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}
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{
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static_assert(noexcept(std::tuple<NoExceptDefault>()), "");
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static_assert(noexcept(std::tuple<NoExceptDefault, NoExceptDefault>()), "");
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static_assert(!noexcept(std::tuple<ThrowingDefault, NoExceptDefault>()), "");
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static_assert(!noexcept(std::tuple<NoExceptDefault, ThrowingDefault>()), "");
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static_assert(!noexcept(std::tuple<ThrowingDefault, ThrowingDefault>()), "");
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}
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#ifndef _LIBCPP_HAS_NO_CONSTEXPR
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{
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constexpr std::tuple<> t;
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}
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{
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constexpr std::tuple<int> t;
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assert(std::get<0>(t) == 0);
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}
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{
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constexpr std::tuple<int, char*> t;
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assert(std::get<0>(t) == 0);
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assert(std::get<1>(t) == nullptr);
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}
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{
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// Check that the SFINAE on the default constructor is not evaluted when
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// it isn't needed. If the default constructor is evaluted then this test
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// should fail to compile.
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IllFormedDefault v(0);
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std::tuple<IllFormedDefault> t(v);
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}
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#endif
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}
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