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#include <iostream>
#include <type_traits>
 
using namespace std;
 
int main() {
	auto f = [](auto x, auto y = std::decay_t<decltype(x)>{}) {
		;
	};
 
	f(0);
 
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:11:5: error: no match for call to ‘(main()::<lambda(auto:1, auto:2)>) (int)’
  f(0);
     ^
prog.cpp:7:58: note: candidate: template<class auto:1, class auto:2> main()::<lambda(auto:1, auto:2)>::operator decltype (((const main()::<lambda(auto:1, auto:2)>*)((const main()::<lambda(auto:1, auto:2)>* const)0u))->operator()(static_cast<auto:1&&>(<anonymous>), static_cast<auto:2&&>(<anonymous>))) (*)(auto:1, auto:2)() const
  auto f = [](auto x, auto y = std::decay_t<decltype(x)>{}) {
                                                          ^
prog.cpp:7:58: note:   template argument deduction/substitution failed:
prog.cpp:11:5: note:   couldn't deduce template parameter ‘auto:2’
  f(0);
     ^
prog.cpp:7:58: note: candidate: template<class auto:1, class auto:2> main()::<lambda(auto:1, auto:2)>
  auto f = [](auto x, auto y = std::decay_t<decltype(x)>{}) {
                                                          ^
prog.cpp:7:58: note:   template argument deduction/substitution failed:
prog.cpp:11:5: note:   couldn't deduce template parameter ‘auto:2’
  f(0);
     ^

Standard output is empty