[英]ADL related GCC 4.7.2 issue with expression SFINAE
Take the following code, which is characterized by 采用以下代码,其特点是
volume
) 依赖于ADL的特定行为( volume
) namespace Nature {
struct Plant {};
double volume(Plant){ return 3.14; }
}
namespace Industrial {
struct Plant {};
double volume(Plant) { return 100; }
}
namespace SoundEffects {
// A workaround for GCC, but why?
////template<class T> void volume();
template<class aSound>
auto mix(aSound& s) -> decltype(volume(s)*0.1)
{
return volume(s)*.1;
}
struct Samples {
Nature::Plant np;
Industrial::Plant ip;
};
inline double mix(const Samples& s) {
return mix(s.np) + mix(s.ip);
}
}
int main()
{
SoundEffects::Samples s;
assert( mix(s) == 100*.1 + 3.14*.1 );
}
The code as presented (without the template<class T> void volume()
line), VS 2012 and clang 3.5 compiles successfully, and runtime is as expected. 显示的代码(没有template<class T> void volume()
行),VS 2012和clang 3.5成功编译,运行时是预期的。 However, GCC 4.7.2 says: 但是,GCC 4.7.2说:
template-function-overload.cpp: In substitution of 'template<class aSound> decltype ((volume(s) * 1.0000000000000001e-1)) SoundEffects::mix(aSound&) [with aSound = SoundEffects::Samples]':
template-function-overload.cpp:46:4: required from here
template-function-overload.cpp:23:9: error: 'volume' was not declared in this scope
template-function-overload.cpp:23:9: note: suggested alternatives:
template-function-overload.cpp:9:11: note: 'Nature::volume'
template-function-overload.cpp:14:11: note: 'Industrial::volume'
With the extra template volume
line, all three compile and run fine. 使用额外的template volume
行,所有三个编译并运行正常。
So, there is clearly a compiler defect here. 所以,这里显然存在编译器缺陷。 My question is, which compiler is the defective one? 我的问题是,哪个编译器是有缺陷的? And which C++ standard is being violated? 哪个C ++标准被违反了?
This was a bug which was fixed since GCC 4.8 . 这是自GCC 4.8以来修复的错误。 Here's a simplified version of the code that gives the same error: 这是代码的简化版本 ,它提供了相同的错误:
template<class T>
auto buzz(T x) -> decltype(foo(x));
void buzz(int);
int main() {
buzz(5); // error: 'foo' was not declared in this scope
}
In mix(s)
, both overloads of SoundEffects::mix
are compiled into the set of candidate overloads via ADL ( SoundEffects
is an associated namespace of SoundEffects::Sample
). 在mix(s)
, SoundEffects::mix
两个重载都通过ADL编译成候选重载集( SoundEffects
是SoundEffects::Sample
的关联命名空间)。 The function template overload is evaluated for viability. 评估函数模板重载的可行性。 The error occurs because volume(s)
cannot be resolved to a suitable overload either via pure unqualified lookup or ADL for Sample
. 错误发生,因为volume(s)
不能被解析为一个合适的过载或者通过纯不合格查找或ADL为Sample
。
The reason this should pass is that when lookup fails a substitution failure should occur (since volume(s)
is dependent) and the template should be rejected from overload resolution. 这应该通过的原因是当查找失败时,应该发生替换失败(因为volume(s)
是依赖的),并且应该从重载解析中拒绝模板。 This would leave mix(const Sample&)
as the only viable overload to select. 这将使mix(const Sample&)
成为唯一可行的重载选择。 The fact that there is a hard error is clearly a sign that this GCC version has a faulty SFINAE implementation. 存在硬错误的事实显然表明该GCC版本具有错误的SFINAE实现。
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