See bug 1306450. The following pattern is quite common: if (x === undefined) x = 0; ES6 default arguments use similar bytecode: function f(x = 0) {} IonBuilder has code to optimize TypeSets when using these comparisons, but it doesn't handle this case. After the if-statement, we should be able to remove |undefined| from x's TypeSet, with some analysis.

Is this still a problem? I think I get the correct information for this trivial example. > function f(x) { > if (x === undefined) > x = 0; > return x; > } > > function g(x = 0) { > return x; > } > > f() > g()

This example is probably too trivial. We want good type information even when the call to f or g is not inlined and the method is called with no parameter, undefined and some number like 0. So even when type of x is undefined or a number the type inside the body (or after the if) is still a number.