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#include <functional>
 
template<typename T>
class Value {
    std::function<T()> get;    
public:
 
    template<class Y, typename = decltype(std::declval<Y&>()())>
    Value(Y lambda ) : get( std::move( lambda ) )  {}
};
 
int main()
{
	Value<double> d1 = [] { return 1.; };
    Value<double> d2 = 123;
 
}

prog.cpp:15:19: error: no viable conversion from 'int' to 'Value<double>'
    Value<double> d2 = 123;
                  ^    ~~~
prog.cpp:4:7: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'int' to 'const Value<double> &' for 1st argument
class Value {
      ^
prog.cpp:4:7: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'int' to 'Value<double> &&' for 1st argument
class Value {
      ^
prog.cpp:9:5: note: candidate template ignored: substitution failure [with Y = int]: called object type 'int' is not a function or function pointer
    Value(Y lambda ) : get( std::move( lambda ) )  {}
    ^
1 error generated.

Standard output is empty