为何C++模板实参推导无法处理大括号初始化列表？

问题背景 #

已有相关讨论涉及类似现象，但本文重点探究其背后的原理。以下代码旨在通过模板实现结构体成员的反射功能：

示例代码 #

template <typename T>
constexpr auto mem_vars = [] { static_assert(!sizeof(T), "mem_vars is not specialized"); };

template <typename T, typename M>
struct mem_refl {
  M T::*mem_ptr;
  std::string_view name;
};

template <typename T, typename M>
mem_refl(M T::*, std::string_view) -> mem_refl<T, M>;

template <typename T, typename... M>
constexpr std::tuple<mem_refl<T, M>...> make_reflect(mem_refl<T, M>... m) {
  // todo: static asssert that all the fields have been specified
  return {m...};
}

struct foo {
  std::string a;
  int b;
};

template <>
constexpr auto mem_vars<foo> = make_reflect({&foo::a, "a"} /*, mem_refl{&foo::b, "b"}*/);

编译错误详情 #

未显式指定模板参数T时的错误 #

编译时触发推导/替换错误：

serialization.cpp|51 col 44| error: no matching function for call to ‘make_reflect(<brace-enclosed initializer list>)’
||    51 | constexpr auto mem_vars<foo> = make_reflect({&foo::a, "a"} /*, mem_refl{&foo::b, "b"}*/);
||       |                                ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
serialization.cpp|40 col 41| note: candidate: ‘template<class T, class ... M> constexpr std::tuple<mem_refl<T, M>...> make_reflect(mem_refl<T, M>...)’
||    40 | constexpr std::tuple<mem_refl<T, M>...> make_reflect(mem_refl<T, M>... m) {
||       |                                         ^~~~~~~~~~~~
serialization.cpp|40 col 41| note:   template argument deduction/substitution failed:
serialization.cpp|51 col 44| note:   couldn’t deduce template parameter ‘T’
||    51 | constexpr auto mem_vars<foo> = make_reflect({&foo::a, "a"} /*, mem_refl{&foo::b, "b"}*/);
||       |                                ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

显式指定T为foo时的错误 #

按提示显式指定模板参数后，出现另一个困惑性错误：

serialization.cpp|51 col 49| error: too many arguments to function ‘constexpr std::tuple<mem_refl<T, M>...> make_reflect(mem_refl<T, M>...) [with T = foo; M = {}]’
||    51 | constexpr auto mem_vars<foo> = make_reflect<foo>({&foo::a, "a"} /*, mem_refl{&foo::b, "b"}*/);
||       |                                ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
serialization.cpp|40 col 41| note: declared here
||    40 | constexpr std::tuple<mem_refl<T, M>...> make_reflect(mem_refl<T, M>... m) {
||       |                                         ^~~~~~~~~~~~

核心疑问 #

当前作用域中仅存在一个make_reflect函数模板，仅接受mem_refl类型参数。我能理解编译器无法直接从大括号初始化列表推导mem_refl类型的情况，但如果显式写出make_reflect(mem_refl{&foo::a, "a"})，类模板推导指南就能正常生效。因此推测问题根源在于模板参数推导与类模板推导的执行顺序，但不确定这是语言特性限制还是用法错误。

内容的提问来源于stack exchange，提问作者Sergey Kolesnik