<hash_map>: Consistently use `int = 0` SFINAE

[StephanTLavavej]

Over the years, we settled on a consistent pattern for SFINAE: default template arguments of the form enable_if_t<CONDITION, int> = 0. This works in a wide variety of situations, including when we have an overload set where SFINAE makes exactly one overload viable. There are a few remaining occurrences of SFINAE that don't follow this pattern.

In <hash_map>, these occurrences are:

STL/stl/inc/hash_map

Line 193 in 28ec9a3

template <class _Valty, class = enable_if_t<is_constructible_v<value_type, _Valty>>>

STL/stl/inc/hash_map

Line 198 in 28ec9a3

template <class _Valty, class = enable_if_t<is_constructible_v<value_type, _Valty>>>

STL/stl/inc/hash_map

Line 376 in 28ec9a3

template <class _Valty, class = enable_if_t<is_constructible_v<value_type, _Valty>>>

STL/stl/inc/hash_map

Line 381 in 28ec9a3

template <class _Valty, class = enable_if_t<is_constructible_v<value_type, _Valty>>>

This change is currently blocked by a compiler bug. We need to provide a reduced test case to the compiler front-end team.

(Split from #187.)

[jpjjulie]

I can try to work on this.

[StephanTLavavej]

Reported the compiler bug as DevCom-848104 "MSVC incorrectly rejects enable_if_t<Condition, int> = 0 SFINAE":

C:\Temp\HASH>type hash.cpp
#include <type_traits>
#include <utility>
using namespace std;

template <typename Traits>
struct HashBase {
    using value_type = typename Traits::value_type;

    void insert(value_type&&) = delete;
};

template <typename K, typename V>
struct HashTraits {
    using value_type = pair<const K, V>;
};

template <typename K, typename V>
struct HashMap : HashBase<HashTraits<K, V>> {
    using MyBase = HashBase<HashTraits<K, V>>;

#ifdef WORKAROUND1
    using value_type = pair<const K, V>;
#else
    using value_type = typename MyBase::value_type;
#endif

    using MyBase::insert;

#ifdef WORKAROUND2
    template <typename T, class = enable_if_t<is_constructible_v<value_type, T>, int>>
#else
    template <typename T, enable_if_t<is_constructible_v<value_type, T>, int> = 0>
#endif

    void insert(T&&) {}
};

struct Movable {
    Movable() = default;
    Movable(Movable&&) = default;
    Movable& operator=(Movable&&) = default;

    Movable(const Movable&) = delete;
    Movable& operator=(const Movable&) = delete;
};

int main() {
    HashMap<Movable, int> hm;
    pair<Movable, int> p(Movable{}, 3);
    hm.insert(move(p));
}

C:\Temp\HASH>cl /EHsc /nologo /W4 /c hash.cpp
hash.cpp
hash.cpp(50): error C2280: 'void HashBase<HashTraits<K,V>>::insert(std::pair<const K,V> &&)': attempting to reference a deleted function
        with
        [
            K=Movable,
            V=int
        ]
hash.cpp(9): note: see declaration of 'HashBase<HashTraits<K,V>>::insert'
        with
        [
            K=Movable,
            V=int
        ]
hash.cpp(9): note: 'void HashBase<HashTraits<K,V>>::insert(std::pair<const K,V> &&)': function was explicitly deleted
        with
        [
            K=Movable,
            V=int
        ]

C:\Temp\HASH>cl /EHsc /nologo /W4 /c /DWORKAROUND1 hash.cpp
hash.cpp

C:\Temp\HASH>cl /EHsc /nologo /W4 /c /DWORKAROUND2 hash.cpp
hash.cpp

C:\Temp\HASH>clang-cl -m32 /EHsc /nologo /W4 /c hash.cpp

C:\Temp\HASH>clang-cl -m32 /EHsc /nologo /W4 /c /DWORKAROUND1 hash.cpp

C:\Temp\HASH>clang-cl -m32 /EHsc /nologo /W4 /c /DWORKAROUND2 hash.cpp

C:\Temp\HASH>