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template <class T>
auto f(T v) -> decltype(g(v));
 
int g(int) { return 0; }
 
template <class T>
auto f(T v) -> decltype(g(v))
{
  return g(v) + 1;
}
 
int main()
{
  return f(0);
}
 

prog.cpp: In function 'int main()':
prog.cpp:14:13: error: no matching function for call to 'f(int)'
   return f(0);
             ^
prog.cpp:7:6: note: candidate: template<class T> decltype (g(v)) f(T)
 auto f(T v) -> decltype(g(v))
      ^
prog.cpp:7:6: note:   template argument deduction/substitution failed:
prog.cpp: In substitution of 'template<class T> decltype (g(v)) f(T) [with T = int]':
prog.cpp:14:13:   required from here
prog.cpp:2:26: error: 'g' was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
 auto f(T v) -> decltype(g(v));
                          ^
prog.cpp:4:5: note: 'int g(int)' declared here, later in the translation unit
 int g(int) { return 0; }
     ^

Standard output is empty