C++17 if statement with initializer but no condition

In c++17 you can do the following:

if ( init-statement(optional); condition )

according to https://en.cppreference.com/w/cpp/language/if

However my compiler (vs2019) allows

if ( init-statement )

where the init-statement is also the condition. I can't find that this is documented anywhere, is this actually guaranteed to work?