c++ - 是否允许在模板特化时从 int 转换为 long (此代码应该编译)？

Question

看看这个片段:

template <long>
struct Bar { };

template <typename>
struct Foo;

template <int X>
struct Foo<Bar<X>> { };

Foo<Bar<0>> x;

Bar有long参数，但是Foo有int 。 Clang-4/5/6 编译了这个，但 GCC-6/7/8 没有:

error: aggregate ‘Foo<Bar<0> > x’ has incomplete type and cannot be defined

哪个编译器是正确的？

Answer 1

据我了解，这些部分推导应该失败:

• 对于模板类:

14.3.3 Template template arguments [temp.arg.template]

3. A template-argument matches a template template-parameter P when P is at least as specialized as the template-argument A. If P contains a parameter pack, then A also matches P if each of A’s template parameters matches the corresponding template parameter in the template-parameter-list of P. Two template parameters match if they are of the same kind (type, non-type, template), for non-type template-parameters, their types are equivalent (14.5.6.1), and for template template-parameters, each of their corresponding template-parameters matches, recursively.

等价定义为:

14.5.6.1 Function template overloading [temp.over.link]

5. Two expressions involving template parameters are considered equivalent if two function definitions containing the expressions would satisfy the one-definition rule (3.2)

所以自从替换 int与 long函数定义中将不满足单一定义规则模板参数将不匹配且 Foo不会选择专业。

• 对于模板函数:

14.8.2.5 Deducing template arguments from a type [temp.deduct.type]

17. If P has a form that contains <i>, and if the type of i differs from the type of the corresponding template parameter of the template named by the enclosing simple-template-id, deduction fails. If P has a form that contains [i], and if the type of i is not an integral type, deduction fails.

template<int i> class A { /* ... */ };

template<short s> void f(A<s>);
void k1() {
    A<1> a;
    f(a); // error: deduction fails for conversion from int to short
    f<1>(a); // OK
}