[[CanPut]] semantics

[Brendan Eich]

Oliver Hunt wrote:
> My reading of [[CanPut]] implies that it only checks a single level of the prototype chain.  This seems incorrect to me but I thought I'd check whether it is in fact intentional, or whether it's a spec bug.

See step 4 and later.

/be

ES5.1 8.12.4 [[CanPut]] (P)

When the [[CanPut]] internal method of O is called with property name P, 
the following steps are taken:

1. Let desc be the result of calling the [[GetOwnProperty]] internal 
method of O with argument P.
2. If desc is not undefined, then
  a. If IsAccessorDescriptor(desc) is true, then
   i. If desc.[[Set]] is undefined, then return false.
   ii. Else return true.
  b. Else, desc must be a DataDescriptor so return the value of 
desc.[[Writable]].
3. Let proto be the [[Prototype]] internal property of O.
4. If proto is null, then return the value of the [[Extensible]] 
internal property of O.
5. Let inherited be the result of calling the [[GetProperty]] internal 
method of proto with property name P.
6. If inherited is undefined, return the value of the [[Extensible]] 
internal property of O.
7. If IsAccessorDescriptor(inherited) is true, then
  a. If inherited.[[Set]] is undefined, then return false.
  b. Else return true.
8. Else, inherited must be a DataDescriptor
  a. If the [[Extensible]] internal property of O is false, return false.
  b. Else return the value of inherited.[[Writable]].

Host objects may define additional constraints upon [[Put]] operations. 
If possible, host objects should not
allow [[Put]] operations in situations where this definition of 
[[CanPut]] returns false.