Template arguments deduction for function parameter pack followed by other parameters

Question

Is the deduction for f1 and f2 ill-formed?

template<class... T, class U>
void f1(T..., U){}

template<class... T>
void f2(T..., int){}

int main()
{
    f1(1);
    f2(1);
    return 0;
}

g++ accepts both, clang only accepts f2, and msvc rejects both.

Related standard wording:

[temp.deduct.call]

When a function parameter pack appears in a non-deduced context ([temp.deduct.type]), the type of that parameter pack is never deduced.

[temp.deduct.type]p5

The non-deduced contexts are:

• A function parameter pack that does not occur at the end of the parameter-declaration-list.

So it seems that MSVC is correct in rejecting both?

Does it mean that any instantiation of the templates will be ill-formed, even if you specify the template args explicitly?

f1<int>(1, 2); // ill-formed?
f2<int>(1, 2); // ill-formed?

If that's the case, why allow such declarations at first place?

Answer 1

There's a DR for this specific issue DR1388. Aparently, it seems that GCC and CLANG haven't implemented it yet CLANG DR1388.

Does it mean that any instantiation of the templates will be ill-formed, even if you specify the template args explicitly?

f1<int>(1, 2); // ill-formed?
f2<int>(1, 2); // ill-formed?

If that's the case, why allow such declarations at first place?

No if you specify explicitly the template arguments, no deduction occurs and as such the code showed above is legal.