The boolean conversion operator for std::basic_istream is explicit. This means that instances of the type will not implicitly become a bool but can be converted to one explicitly, for instance by typing bool(infile).
Explicit boolean conversion operators are considered for conditional statements, i.e. the expression parts of if, while etc. More info about contextual conversions here.
However, a return statement will not consider the explicit conversion operators or constructors. So you have to explicitly convert that to a boolean for a return.