Why C++ template accepting array is not more specialized than one accepting pointer (bis)?

问题

_{In reference to this question, which has indeed the same title but for which I found an answer in the standard. I have continued to dig the subject and finaly find out an example code for which this answer does not apply.}

Let's consider this piece of code:

template<class T> void func(T* buf);           //template I
template<size_t N> void func(char (&buf) [N]); //template II

void g(char (&buf)[3])
   {
   func(buf) //Error: ambiguous function call (Clang, GCC, ICC, MSVC)
   }

According to the partial ordering rules in [temp.func.order] and [temp.deduct.partial], template II shall be more specialized than template I if one interpreted this rules through the execution of this piece of code:

template <class T> void func1(T* buf) {}
template <std::size_t N> void func2(char (&buf)[N]) {}

struct invented_T{};
constexpr std::size_t invented_N=42;

void is_template_I_more_specialized(invented_T* buf)
  {
  func2(buf);
  //DO NOT COMPILE
  // => template I is not more specialized than func2
  }

void is_template_II_more_specialized(char (&buf)[invented_N])
  {
  func1(buf);
  //DO COMPILE
  // => template II is more specialized than func1
  }

So according to this interpretation, template II should be more specialized. Why would it not be the case?

回答1:

As n.m. pointed out in the comment, the reason is that type T* cannot be deduced from type char (&buf)[invented_N].

In is_template_II_more_specialized, an extra array-to-pointer conversion is applied according to [temp.deduct.call]/2.1:

If P is not a reference type:

If A is an array type, the pointer type produced by the array-to-pointer standard conversion is used in place of A for type deduction; otherwise,

...

This rule only applies for deducing template arguments from a function call. For deducing template arguments during partial ordering, there is no such conversion applying.

_{Conversions that can be applied during partial ordering are described in [temp.deduct.partial]/5,6,7.}

来源：https://stackoverflow.com/questions/48429659/why-c-template-accepting-array-is-not-more-specialized-than-one-accepting-poin

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c++

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language-lawyer

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