Partial Specialization of Variable Templates

Question

I know that I can partially specialize class templates, and I know that I cannot partially specify function templates.

What about variable templates ? I can't find documentation on whether they can be partially specialized.

Answer 1

Yes, according to [temp.arg.template]/2 :

Any partial specializations associated with the primary class template or primary variable template are considered when a specialization based on the template template-parameter is instantiated. ...

And to [temp.expl.spec]/7 :

... the placement of partial specialization declarations of class templates, variable templates , member class templates of non-template classes, static data member templates of non-template classes, member class templates of class templates, etc. ...

It also mentioned in [constraints.namespace.std]/3 :

The behavior of a C++ program is undefined if it declares an explicit or partial specialization of any standard library variable template , except where explicitly permitted by the specification of that variable template.

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Not to mention that all major compilers (Clang, GCC and MSVC) have no problems with them:

template <int x, int y>
constexpr int foo = -1;

template <>
constexpr float foo<1, 0> = 1.0;

template <int x>
constexpr double foo<1, x> = 1.1;

int main()
{
    static_assert(foo<0, 0> == -1, "");
    static_assert(foo<0, 1> == -1, "");
    static_assert(foo<1, 0> == 1.0, "");
    static_assert(foo<1, 1> == 1.1, "");
}

https://godbolt.org/z/R1c1zo