Defining fstream inside a 'if' conditional

In an answer there was the following code:

if (std::ifstream input("input_file.txt"))
  ;

Which seems convenient, limiting the scope of the 'input' variable to where it's confirmed to be valid, however neither VS2015 nor g++ seems to compile it. Is it some compiler specific thing or does it require some extra flags?

In VS2015 the IDE highlights "std::ifstream" and the "input_file.txt" as well as the last parentheses. "std::ifstream" is flagged with "Error: a function type is not allowed here".

VS2015 C++ compiler gives following errors:

• C4430 missing type specifier - int assumed. Note: C++ does not support default-int
• C2059 syntax error: '('

PS. I created new question instead of commenting in that answer since I can't comment.