RFR 8191802: Upward projection result is A<? extends Number> instead of A<? super Integer>

[Maurizio Cimadamore]

This is an updated version of the webrev:

http://cr.openjdk.java.net/~mcimadamore/8191802-v2/

I realized that there were further discrepancies w.r.t. the spec text, 
so I handled them all at once:

* the spec has different treatment for when a type argument is a 
wildcard and for when it's a regular type - the implementation was 
treating both wildcards and regular types in the same way, which led to 
inconsistencies (see JDK-8191893). I now defined separate visitors for 
the outer projection and type argument projection parts, so that the 
code should be easier to follow.

* not all fresh types created during a projection overrode the 
'needStripping' method, which could lead to issue with type annotation 
processing

* the fact that TypeProjection extended from StructuralMapping had a 
subtle issue - on the one hand, StructuralMapping has the required logic 
to handle arrays (e.g. map component type and return new array if 
needed) - on the other hand, with projections we need to be careful - if 
the element type has no projection, then projection of the array is also 
undefined. The impl was returning an array of <null> which was then 
causing crashes inside the compiler. This is now called out explicitly.

Maurizio


On 24/11/17 17:39, Maurizio Cimadamore wrote:
>
> Hi,
> please review the fix for JDK-8191802:
>
> http://cr.openjdk.java.net/~mcimadamore/8191802/
>
> This is a conformance issue with local variable type inference - the 
> specification text for upper projection says this (section 4.10.5):
>
> "The upward projection of a type T with respect to a set of restricted 
> type variables is defined as follows:
>
>     [...]
>
>     If T is a parameterized class type or a parameterized interface 
> type, G<A1, ..., An>, then the result is G<A1', ..., An'>, where, for 
> 1 ≤ i ≤ n, Ai' is derived from Ai as follows:
>
>         [...]
>         If Ai is a type that mentions a restricted type variable, then 
> Ai' is a wildcard. Let U be the upward projection of Ai. There are 
> three cases:
> *If U is not Object, and if either the declared bound of the ith 
> parameter of G, Bi, mentions a type parameter of G, or Bi is not a 
> subtype of U, then Ai' is an upper-bounded wildcard, ? extends U.*
>             Otherwise, if the downward projection of Ai is L, then Ai' 
> is a lower-bounded wildcard, ? super L.
>             Otherwise, the downward projection of Ai is undefined and 
> Ai' is an unbounded wildcard, ?."
>
> The text in bold is not implemented accurately by javac. What javac 
> used to do was simply to throw away the upper bound and favor the 
> lower bound if the upper was Object.
>
> The spec text is much more subtle and precise, allowing javac to throw 
> away upper bounds that do not add any extra information w.r.t. 
> declared bounds.
>
> As a result of this change, there are few places where the compiler 
> used to infer A<? extend B> (where B was same type as declared bound) 
> and now it infers A<?> as per spec - this caused few changes in the 
> jshell test TypeNameTest.
>
> I've added a lvti harness test to verify the assertions in the above 
> paragraph.
>
> Cheers
> Maurizio
>

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