• A type guard of the form expr1 || expr2
o when true, narrows the type of x to T1 | T2, where T1 is the type of x narrowed by expr1 when true, and T2 is the type of x narrowed by expr1 when false and then by expr2 when true, or
o when false, narrows the type of x by expr1 when false and then by expr2 when false.
