Aside from the fact (as has already been pointed out in previous posts) that your reference to convertToYen in main is a function declaration, the code should not compile with no value being returned from convertToYen as long as it has a return type of double.
Sad, that is. Apparently all the standard has to say about it is:
Flowing off the end of a function is equivalent to a return with no value; this results in undefined
behavior in a value-returning function.
And the standard doesn't require a diagnostic for undefined behavior, although, since it is trivially detectable in cases where there are no return statements in the function body, one would expect a warning as a QOI issue.
MSVC++ generates an error by default for functions that can be proved don't return values and a warning for cases where it just may be the case. (That behavior can, of course, be adjusted.)