Visual C ++ 2015 gives warning C4552 when using decltype (auto)

Question

I think I encountered an error in Visual C ++ 2015, but I want to be sure. Consider this snippet:

template < typename T > decltype( auto ) f( T param ) { return param + 1; } int main() { auto i = f( 10 ); return 0; } 

Visual C ++ 2015 gives this warning in the return statement:

warning C4552: '+': operator is not valid; expected operator with side effect

although it does not seem to affect the resulting code. Is this a compiler error?

Answer 1

This seems to be a mistake. There is an open error report here:

https://connect.microsoft.com/VisualStudio/feedback/details/1468881/decltype-auto-causes-unnecessary-warnings

Answer 2

As mentioned in the Csq answer , the observed behavior is pending a Microsoft Connect report . This question has not been rated yet.

To get around the problem ¹⁾ you can include parentheses around the expression:

 template < typename T > decltype( auto ) f( T param ) { return ( param + 1 ); } 

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¹⁾ Tested using Visual Studio 2015 Community Edition

Answer 3

As pointed out by Microsoft itself here , this could be a “bug”. Try:

 decltype(auto) f(T param) { return (param + 1); } 

Or simply suppress this war, because Microsoft probably won't fix it because of their “mistaken” excuses.