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"candidate template ignored: could not match ..." for template function argument to template function

By : MichaelR1965
Date : September 22 2020, 01:00 PM
I wish did fix the issue. If you specify a template argument list for a function call, as in compose, then this list is considered partial if there are more template parameters than arguments.
Calling the function still does template argument deduction on the remaining template parameters.
code :
template<typename T, typename... S>
void compose(typename std::type_identity<compose_fn_t<T, S...>>::type fn) {};
template<typename T>
struct type_identity {
    using type = T;
template<typename T, typename... S, typename F>
void compose(F&& f) {
    compose_fn_t<T, S...> fn{std::forward<F>(f)};
    // Use fn as before
compose(std::function{[] (B b, C c) -> A { return {}; }});
template<typename F>
void compose(F&& f) {
    auto fn = std::function{std::forward<F>(f)};
    // use fn here as before
compose([] (B b, C c) -> A { return {}; });

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"Template argument for template template parameter must be a class template or type alias template"

By : sarat sahoo
Date : March 29 2020, 07:55 AM
around this issue You have a template template parameter. You must pass a template as its argument. You instead pass a template instantiation as its argument - which is a concrete class, not a template (all its parameters are bound).
code :
template <template<typename> typename X>
class WantsTemplate {};

template <typename>
class ATemplate {};

WantsTemplate<ATemplate> wt1;  // OK
WantsTemplate<ATemplate<int> > wt2;  // not OK

"use of class template requires template argument list" error, even though template of template is used, there

By : Don Ky
Date : March 29 2020, 07:55 AM
I wish this helpful for you context/ultimate wish (though barely relevant to the question) , U requires an argument (see its declaration)
code :
template < typename T2 >
    struct U
template < typename T >
    struct UnaryPairMaker : Unarify< MakePair, ToPlaceLeftT >::U
template < typename T >
    struct UnaryPairMaker : Unarify< MakePair, ToPlaceLeftT >::template U<T>

C++ template argument deduction is failing with error "candidate template ignored" when arguments include std:

By : Syarifudin
Date : March 29 2020, 07:55 AM
Any of those help Type deduction does not consider implicit conversions, that happens later during overload resolution. If you were to pass a std::function directly, it would deduce properly. It can't implicitly construct a std::function from your lambda without the type of the std::function to know whether it's possible. I know to us it looks like there is enough information for the compiler to properly deduce the types involved, but this is a constraint.

C++ template overload resolution called from template function pick candidate declared after template instantiation

By : koble73
Date : March 29 2020, 07:55 AM
this will help [temp.dep.candidate]/1:

Why compiler said "candidate template ignored: couldn't infer template argument 'InputIterator'"?

By : user1834998
Date : March 29 2020, 07:55 AM
this one helps. [The code in the question is neither minimal nor reproducible.]
The issue is that parameters of type cls::type are not deducible. This is the exact pattern you have in:
code :
template <typename InputIterator>
Vector(typename __isInputIterator<InputIterator>::__result, typename __isInputIterator<InputIterator>::__result, const allocator &);
template <typename InputIterator, 
         typename __isInputIterator<InputIterator>::__result* = nullptr>
Vector(InputIterator, InputIterator, const allocator & = allocator{});
template <typename InputIterator,
        std::enable_if_t<__isInputIterator<InputIterator>::__result::value>* x = nullptr>
 Vector(InputIterator, InputIterator, const allocator & = allocator{});
#include <memory>
#include <type_traits>
struct InputIterator {
    constexpr static bool isInputIterator {true};
struct __falseType 
    static constexpr bool value{false};
struct __trueType 
    static constexpr bool value{true};

template <typename, bool>
struct __InputIteratorInferringAuxiliary {
    using __type = __falseType;
template <typename Iterator>
struct __InputIteratorInferringAuxiliary<Iterator, true> {
    using __type = __trueType;

struct NotIterator {
static constexpr bool isInputIterator = false;
template <typename T>
struct __IteratorTraits {
    using iteratorTag = NotIterator;

struct RandomAccessIterator {
        static constexpr bool isInputIterator = true;


template <typename T>
struct __IteratorTraits<T *> {
    using sizeType = unsigned long;
    using differenceType = long;
    using valueType = T;
    using reference = valueType &;
    using constReference = const valueType &;
    using rightValueReference = valueType &&;
    using pointer = valueType *;
    using constPointer = const valueType *;
    using iteratorTag = RandomAccessIterator;//isInputIterator tag in RandomAccessIterator class is true

template <typename Iterator>
struct __isInputIterator {
    using __result = typename __InputIteratorInferringAuxiliary<Iterator,
template <class T, class allocator=std::allocator<T>>
class Vector
    template <typename InputIterator,
            std::enable_if_t<__isInputIterator<InputIterator>::__result::value>* x = nullptr>
     Vector(InputIterator, InputIterator, const allocator & = allocator{});
       T* begin();
       T* end();
int main()
    int arr[] {1, 2, 3};
Vector<int> vec(std::begin(arr), std::end(arr));//candidate template ignored: couldn't infer template argument 'InputIterator'
Vector<int> vec2(1,2);//candidate template ignored: couldn't infer template argument 'InputIterator'

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