[英]Function template modifies parameter declared with top-level const: clang bug?
The code below compiles correctly on clang 3.8.1-1
on ArchLinux. 下面的代码clang 3.8.1-1
在ArchLinux的clang 3.8.1-1
上正确编译。
Is this clang
bug? 这是clang
虫吗?
gcc
issue correct warning/error on this. 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;
}
Here how I compile: 这里是我的编译方式:
[nmmm@zenbook HM3]$ clang x.cc -std=c++11 -lstdc++ -Wall -Wpedantic -Wconversion
clang
and c++14
- same result. clang
和c++14
相同的结果。
[nmmm@zenbook HM3]$ clang x.cc -std=c++14 -lstdc++ -Wall -Wpedantic -Wconversion
Here is gcc output: 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;
~~~~~~^~~
Yes, this is a bug in clang; 是的,这是Clang中的错误; filed at https://llvm.org/bugs/show_bug.cgi?id=30365 . 归档于https://llvm.org/bugs/show_bug.cgi?id=30365 。
The nature of the bug is that in a class template member function definition appearing outside ( [class.mfct] /1) the class template, with the type of a parameter dependent on the class template parameters, clang uses the parameter type of the declaration rather than the parameter type of the definition where they differ in topmost cv-qualification. 该错误的本质是,在类模板成员函数定义出现在类模板之外( [class.mfct] / 1)时,参数的类型取决于类模板参数,clang使用声明的参数类型而不是定义的参数类型,它们的最高简历要求不同。 Simplified example: 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); }
Per [dcl.fct] /5 the type of i
within the definition of A<X>::f
is int const
( Use of 'const' for function parameters ): 每[dcl.fct] / 5在A<X>::f
定义内的i
的类型为int const
( 对函数参数使用'const' ):
5 - [...] After producing the list of parameter types, any top-level cv-qualifiers modifying a parameter type are deleted when forming the function type. 5-[...]生成参数类型列表后,在形成函数类型时会删除所有修改参数类型的顶级cv限定词 。 [...] [ Note: This transformation does not affect the types of the parameters. [...] [ 注意:此转换不会影响参数的类型。 [...] — end note ] [...] — 尾注 ]
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