c – 隐式转换产生“错误：获取临时地址”(GCC vs clang)

error: taking address of temporary

我可以想象上面的错误有意义的典型场景,但在我的情况下,我没有遇到问题.重现错误的缩小(设计)示例如下：

#include <cstddef>

#include <type_traits>

enum class Enum : std::size_t {};

struct Pod {
  std::size_t val;

  constexpr operator Enum() const {
    return static_cast<Enum>(val);
  }
};

template<std::size_t N>
constexpr void foo() {
  using Foo = std::integral_constant<Enum,Pod{N}>;
  // [GCC] error: taking address of temporary [-fpermissive]
}

int main() {
  foo<2>();
}

为什么GCC 8.2在这里抱怨？ Clang 6.0很高兴(see goldbolt.org).

请注意,GCC存在第二个错误,可能有助于分析问题.我也不明白：

error: no matching function for call to Pod::operator Enum(Pod*)

GCC 8.2的完整输出读取

<source>: In instantiation of 'constexpr void foo() [with long unsigned int N = 2]':

<source>:22:10:   required from here

<source>:17:50: error: taking address of temporary [-fpermissive]

   using Foo = std::integral_constant<Enum,Pod{N}>;

                                                  ^

<source>:17:50: error: no matching function for call to 'Pod::operator Enum(Pod*)'

<source>:10:13: note: candidate: 'constexpr Pod::operator Enum() const'

   constexpr operator Enum() const {

             ^~~~~~~~

<source>:10:13: note:   candidate expects 0 arguments,1 provided

Compiler returned: 1