如何检查给定参数类型，隐式使用“operator ()”是否会产生一个最佳可行候选者？

Question

据我了解，函数名称使用的结果可能是以下之一：

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1. 没有（最佳）可行的函数 \xe2\x80\x94 重载解析失败。子结果是：\n
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   1. 没有候选人。
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   2. There are some candidates, just none are viable.
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2. There is exactly one best viable function \xe2\x80\x94 overload resolution succeeds. The selected overload is then either\n
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   1. OK \xe2\x80\x94 the overall call is well-formed.
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   2. not OK (= deleted, protected/private or, perhaps, something else) \xe2\x80\x94 the overall call is ill-formed.
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3. There are more than one best viable functions \xe2\x80\x94 overload resolution fails with ambiguity.
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The question is: How to reliably tell apart outcome #2.2 (at least some of its cases) from outcomes #1.2 and #3 (at least one of them) in the case of implicit usage of operator () (i.e. c(a...)) by means of a type trait that accepts the types of the arguments (including c) to be used in the call?

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(I\'m not interested in outcomes #1.1 and #2.1 as I know that #1.1 does not hold in my particular use case and #2.1 is easily detectable through SFINAE.)

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---

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A specific example. How to implement a type trait that looks something like the following

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/// Would `c(a...)` result in exactly one best viable candidate?\n/// (Where `decltype(c)`, `decltype(a)...` are `C`, `A...`, respectively.)\ntemplate<class C, typename... A>\ninline constexpr bool has_exactly_one_best_viable_call_candidate;\n

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so the following asserts hold?

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struct WithNoViable {\n    void operator ()(void *);\n};\n\nstruct WithDeleted {\n    void operator ()(long) = delete;\n};\n\nstruct WithAmbiguity {\n    void operator ()(long);\n    void operator ()(long long);\n};\n\nstatic_assert(!has_exactly_one_best_viable_call_candidate<WithNoViable, int>);\nstatic_assert( has_exactly_one_best_viable_call_candidate<WithDeleted, int>);\nstatic_assert(!has_exactly_one_best_viable_call_candidate<WithAmbiguity, int>);\n

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Note that in general nothing is known about the types of parameters nor arguments.

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