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

Question

With reference to the following code

#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);
}

This works on clang but does not work on gcc, when I change the type of the partial specialization to accept std::size_t in the index sequence however it works.

Who is right? Clang or gcc?

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See this in action here https://wandbox.org/permlink/5YkuimK1pH3aKJT4

Answer 1

gcc is right. This is exactly [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 ]

Mirroring the example and simplifying the original question:

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
}

I think this is clang bug 16279