// N4606 Committee Draft, Standard for Programming Language C++
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4606.pdf 
#define msg "p217.cpp(100)8.4.2 Explicitly-defaulted functions [dcl.fct.def.default]"

#include <iostream>

struct S {
  constexpr S() = default; // ill-formed: implicit S() is not constexpr
  S(int a = 0) = default; // ill-formed: default argument
  void operator=(const S&) = default; // ill-formed: non-matching return type
  ~S() throw(int) = default; // deleted: exception specification does not match
  private:
  int i;
  S(S&); // OK: private copy constructor
};
S::S(S&) = default; // OK: defines copy constructor

struct trivial {
  trivial() = default;
  trivial(const trivial&) = default;
  trivial(trivial&&) = default;
  trivial& operator=(const trivial&) = default;
  trivial& operator=(trivial&&) = default;
  ~trivial() = default;
};
struct nontrivial1 {

  nontrivial1();
};
nontrivial1::nontrivial1() = default; // not first declaration

int main(){

  std::cout<< msg << std::endl;
  return EXIT_SUCCESS;
}

$ ./cppgl17.sh p217
$ clang++ p217.cpp
p217.cpp:8:3: error: defaulted definition of default constructor is not constexpr
  constexpr S() = default; // ill-formed: implicit S() is not constexpr
  ^
p217.cpp:9:3: error: an explicitly-defaulted constructor cannot have default arguments
  S(int a = 0) = default; // ill-formed: default argument
  ^~~~~~~~~~~~~~~~~~~~~~
p217.cpp:10:8: error: explicitly-defaulted copy assignment operator must return 'S &'
  void operator=(const S&) = default; // ill-formed: non-matching return type
       ^
p217.cpp:11:8: error: ISO C++17 does not allow dynamic exception specifications [-Wdynamic-exception-spec]
  ~S() throw(int) = default; // deleted: exception specification does not match
       ^~~~~~~~~~
p217.cpp:11:8: note: use 'noexcept(false)' instead
  ~S() throw(int) = default; // deleted: exception specification does not match
       ^~~~~~~~~~
       noexcept(false)
p217.cpp:11:3: error: exception specification of explicitly defaulted destructor does not match the calculated one
  ~S() throw(int) = default; // deleted: exception specification does not match
  ^
5 errors generated.

$ g++-7 p217.cpp
p217.cpp:9:18: error: 'S::S(int)' cannot be defaulted
   S(int a = 0) = default; // ill-formed: default argument
                  ^~~~~~~
p217.cpp:11:8: error: ISO C++1z does not allow dynamic exception specifications
   ~S() throw(int) = default; // deleted: exception specification does not match
        ^~~~~
p217.cpp:8:13: error: explicitly defaulted function 'constexpr S::S()' cannot be declared as constexpr because the implicit declaration is not constexpr:
   constexpr S() = default; // ill-formed: implicit S() is not constexpr
             ^
p217.cpp:13:7: note: defaulted default constructor does not initialize 'int S::i'
   int i;
       ^
p217.cpp:10:8: error: defaulted declaration 'void S::operator=(const S&)'
   void operator=(const S&) = default; // ill-formed: non-matching return type
        ^~~~~~~~
p217.cpp:10:8: error: does not match expected signature 'constexpr S& S::operator=(const S&)'