Question

以下代码在ArchLinux上的clang 3.8.1-1上正确编译。

这是clang错误吗？

gcc对此发出正确的警告/错误。

template <class T>
struct BugReproducer{
    using size_type = typename T::size_type;

    int bug1(size_type count);
    int bug2(size_type count) const;
    static int bug3(size_type count);
};

template <class T>
int BugReproducer<T>::bug1(size_type const count){
    // this is a bug. must be not allowed
    count = 5;

    // return is to use the result...
    return count;
}

template <class T>
int BugReproducer<T>::bug2(size_type const count) const{
    // same for const method
    count = 5;
    return count;
}

template <class T>
int BugReproducer<T>::bug3(size_type const count){
    // same for static method
    count = 5;
    return count;
}



struct DummyVector{
    using size_type = int;
};



int main(){
    using BugRepr = BugReproducer<DummyVector>;

    BugRepr reproducer;

    auto a = reproducer.bug1(1);
    auto b = reproducer.bug2(1);
    auto c = BugRepr::bug3(1);

    // return is to use the result...
    return a + b + c;
}

我在这里编译：

[nmmm@zenbook HM3]$ clang x.cc -std=c++11 -lstdc++ -Wall -Wpedantic -Wconversion

clang和c++14 - 结果相同。

[nmmm@zenbook HM3]$ clang x.cc -std=c++14 -lstdc++ -Wall -Wpedantic -Wconversion

这是gcc输出：

[nmmm@zenbook HM3]$ gcc x.cc -std=c++11 -lstdc++ -Wall -Wpedantic -Wconversion
x.cc: In instantiation of ‘int BugReproducer<T>::bug1(BugReproducer<T>::size_type) [with T = DummyVector; BugReproducer<T>::size_type = int]’:
x.cc:46:28:   required from here
x.cc:13:8: error: assignment of read-only parameter ‘count’
  count = 5;
  ~~~~~~^~~
x.cc: In instantiation of ‘int BugReproducer<T>::bug2(BugReproducer<T>::size_type) const [with T = DummyVector; BugReproducer<T>::size_type = int]’:
x.cc:47:28:   required from here
x.cc:22:8: error: assignment of read-only parameter ‘count’
  count = 5;
  ~~~~~~^~~
x.cc: In instantiation of ‘static int BugReproducer<T>::bug3(BugReproducer<T>::size_type) [with T = DummyVector; BugReproducer<T>::size_type = int]’:
x.cc:48:20:   required from here
x.cc:29:8: error: assignment of read-only parameter ‘count’
  count = 5;
  ~~~~~~^~~

Answer 1

是的，这是clang中的一个错误;在https://llvm.org/bugs/show_bug.cgi?id=30365提交。

错误的本质是在类模板成员函数定义中出现（ [class.mfct] / 1）类模板，参数类型依赖于类模板参数，clang使用声明的参数类型而不是最顶层cv-qualification中它们不同的定义的参数类型。简化示例：

template<class T> struct A { void f(typename T::U); };
template<class T> void A<T>::f(typename T::U const i) { i = 1; }
struct X { using U = int; };
int main() { A<X>{}.f(0); }

每 [dcl.fct] / 5 i定义中A<X>::f的类型为int const（Use of 'const' for function parameters）：< / p>

5 - [...]生成参数类型列表后，修改参数的任何顶级 cv-qualifiers 在形成函数类型时删除type。 [...] [注意：此转换不会影响参数的类型。 [...] - 结束记录]

函数模板修改用顶级const：clang bug声明的参数？

1 个答案: