RFR 8191802: Upward projection result is A<? extends Number> instead of A<? super Integer>
Maurizio Cimadamore
maurizio.cimadamore at oracle.com
Mon Nov 27 15:09:23 UTC 2017
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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