Parameter scoping and sloppy eval

parameter-eval.js

// What does this code do?
function f(x = eval("var z = 2; 1"), y = z) {
  return [x, y, z];
}
f();

// Spec says to throw a ReferenceError, since 'z' is not visible either
// to other parameters in the list, or to the function body.
// 
// Results in various engines:
// 
// V8, SpiderMonkey, recent Chakra: ReferenceError: z is not defined
// Chakra (until recently): (early) SyntaxError: 'eval' is not allowed in the default initializer
// JavaScriptCore: returns [1, 2, 2]

// What about this?
function g(x = eval("var z = 2; 1"), y = z) {
  var z;
  return [x, y, z];
}
g();

// Spec, SM, V8: ReferenceError: z is not defined
// JSC: returns [1, 2, undefined]

// Proposal: switch the spec to the JavaScriptCore behavior.

@syg JSC apparently creates the var binding in the parameter scope, as adding a "var z;" to the function body causes the return value to be [1, 2, undefined]. I think that's the semantics I'd want.

@ajklein I still prefer all eval being strict, thus no ability to make new var bindings at all in parameter expressions. I think that aligns closer with the intent of the current semantics. Is there a V8 implementation preference against that? Seems simplest of all possible semantics to me.