不匹配的 `int` 和 `size_t` 不编译的部分模板特化

Question

参考以下代码

#include <utility>
#include <cassert>

template <typename T>
struct Wot;
template <int... ints>
struct Wot<std::index_sequence<ints...>> {};

int main() {
    assert(sizeof(Wot<std::index_sequence<1, 2, 3>>) == 1);
}

这适用于 clang 但不适用于 gcc，当我更改部分专业化的类型以在索引序列中接受 std::size_t 但它有效时。

谁是对的？ Clang 还是 gcc？

在此处查看实际操作 https://wandbox.org/permlink/5YkuimK1pH3aKJT4

Answer 1

gcc 是对的。这正是 [temp.deduct.type]/18:

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. [ Example:
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
}

template<const short cs> class B { };
template<short s> void g(B<s>);
void k2() {
  B<1> b;
  g(b);             // OK: cv-qualifiers are ignored on template parameter types
}
— end example ]

镜像示例并简化原始问题：

template <class T> struct Wot { };

template <int... ints>
void foo(Wot<std::index_sequence<ints...>> ) { }

int main() {
    foo(Wot<std::index_sequence<1, 2, 3>>{}); // error
    foo<1, 2, 3>({}); // ok
}

我认为这是 clang 错误 16279

不匹配的 `int` 和 `size_t` 不编译的部分模板特化

Partial template specialization with mismatching `int` and `size_t` not compiling

c++

templates

partial-specialization

language-lawyer

c++14