improved return type-inference: request for review
Neal Gafter
neal at gafter.com
Thu Feb 26 09:39:37 PST 2009
Your points are well taken, but I don't know how to review the code in the
absence of a specification, as part of the purpose of the review is to
ensure that the code correctly implements the specification.
On Thu, Feb 26, 2009 at 9:32 AM, Maurizio Cimadamore <
Maurizio.Cimadamore at sun.com> wrote:
> Neal Gafter wrote:
>
>> Maurizio-
>>
>> Do you have a draft of the revised JLS specification that describes the
>> new behavior?
>>
> nope :-(
>
> actually it seems like this approach can be quite hard to specify correctly
> (at least this is what I've heard from Alex when I discussed this with him).
>
> Anyway I think that 15.12.2.8 should address several issues, not just this
> one: e.g.
>
> class Foo<X> {}
> <T extends Foo<T>> T m() { ... }
>
> <Z extends Foo<Z>> void test() {
> Z test = m();
> }
>
> this generates the following constraints:
>
> Foo<Z> >> T
> Foo<T> >> T
>
> which in turns, means that:
>
> T = glb(Foo<T>, Foo<Z>) = ???
>
> I don't think that the above glb exists: both Foo<T> and Foo<Z> are class
> types, and neither is a subtype of the other.
> Should we reject this program? I don't think so, but it's not crystal clear
> from the JLS what should we do about this one.
> Both javac and Eclipse accepts it...
>
> Maurizio
>
>>
>> -Neal
>>
>>
>> On Tue, Feb 24, 2009 at 9:43 AM, Maurizio Cimadamore <
>> Maurizio.Cimadamore at sun.com <mailto:Maurizio.Cimadamore at sun.com>> wrote:
>>
>> Hi,
>> this is the fix [1] for an outstanding javac/JLS inference bug:
>> [3] and
>> [4] respectively. Those CRs both have to do with recursive bounds
>> being
>> included in the set of constraints generated by 15.12.2.8 (but not by
>> javac). It's important to fix it both because it's preventing other
>> important inference fixes (6638712) and because it might lead to
>> problems when implementing an item of the project coin (namely
>> 'Concise
>> initialization of generic variables' [2]).
>>
>> Let's try to keep things simple: let's start from the example recently
>> submitted by Martin, which I think it's the simpler test case I've
>> seen
>> so far:
>>
>> class Test<X> {
>>
>> <K extends Test<K>> void m() {}
>>
>> void test() {
>> m();
>> }
>> }
>>
>> When you compile this, the compiler complains with the following
>> message
>> (afaik this affects all compilers, jdk 5, 6, 6-open, 7):
>>
>> TestX.java:6: incompatible types; inferred type argument(s)
>> java.lang.Object do not conform to bounds of type variable(s) K
>> found : <K>void
>> required: void
>> m();
>> ^
>> 1 error
>>
>> The message is somewhat messy, and hard to understand, but the meaning
>> it's simple: the type inferred for K is Object; unfortunately
>> Object is
>> not compatible with K's declared bound; in fact Object should be a
>> subtype of [K:=Object]Test<K> = Test<Object> which is obviously
>> not true.
>>
>> How happened that jaac inferred Object for K? According to JLS
>> 15.12.2.8, the following set of constraints should be derived for the
>> above method call:
>>
>> Test<K> >> K
>>
>> K should then be inferred as glb(Test<K>) = Test<K>.
>>
>> This is unfortunate at best for two reasons: first, the inferred type
>> for K is defined in terms of K itself; secondly, Test<K> does not
>> conform to K's declared bound, as Test<K> <: [K:=Test<K>]Test<K> =
>> Test<Test<K>> does not hold.
>>
>> However, javac impl does not seem to follow JLS here; in fact,
>> javac is
>> deliberately removing any recursive constraint from the set
>> derived from
>> 15.12.2.8. Since there's just one constraint here, this leave us with
>> the answer K = Object, which is slightly better than the solution
>> provided by the JLS as it does not contains further references to K -
>> but still does not satisfy K's declared bound.
>>
>> [Sidebar: there other examples in which javac follows more closely the
>> algorithm described by the JLS - in other words, sometimes javac fails
>> to exclude recursive bounds from the set of generated constraints,
>> because of minor trivial bugs - such bugs are however crucial when it
>> comes to compile the code of some compiler regression tests!]
>>
>> Now the though question: how should JLS deal with this? I think there
>> are at least two available options:
>>
>> 1) Give up inference (which means that javac is right) and inferring
>> Object for K (which then causes a compilation error because Object
>> does
>> not conform to K's bound).
>>
>> 2) Adopt a complex inference scheme that would allow 15.12.2.8 to
>> yield
>> an inferred type that (i) does not depend on K and (ii) respect K's
>> declared (recursive) bound.
>>
>> I went for (2), and here's why: in our running example (and in similar
>> examples we have in our regression codebase), one could be tempted to
>> say: ''this is just rubbish, fix your code, please''. On the other
>> hand,
>> last year I noticed that there are several use cases that would
>> start to
>> fail if we choosed (1):
>>
>> Object value = Enum.valueOf((Class)Test.class, "");
>>
>> where Enum.valueof is defined as follows
>> <T extends Enum<T>> T valueOf(Class<T>, String)
>>
>> In this case we have that T cannot be inferred from actual
>> arguments (as
>> the only argument is raw here). It follows that T must be inferred in
>> 15.12.2.8; here we have the following constraints:
>>
>> T <: Object (for the return type)
>> T <: Enum<T> (declared bound)
>>
>> In other words we are in the same situation as above - only, this
>> coding
>> pattern is way more common (here inferring Object for T would
>> cause the
>> above code to be rejected). At this point you might have a
>> question: how
>> does javac deal with this code? We showed how javac is not capable of
>> handling recursive bounds; the trick here is that javac (very
>> surprisingly) never applies 15.12.2.8 (since this is an unchecked
>> call).
>> Which also means that javac never has to find an answer to the above
>> question. Note that one day javac will be corrected so that 15.12.2.8
>> will be applied even in this case - the fact that javac doesn't apply
>> 15.12.2.8 can be regarded as a bug (one of the biggest consequences of
>> this is CR 6638712).
>>
>> Back to the original question, given the only constraint (derived from
>> 15.12.2.8):
>>
>> Test<K> >> K
>>
>> What type should we infer for K?
>>
>> I have shown that Object is not a viable answer (does not respect K's
>> declared bound). At first it would seem that we should pick a type
>> among
>> the following candidates:
>>
>> -Test<?>
>> -Test<? extends Test<?>>
>>
>> Those types are surely better than Object, but all have the problem of
>> not satisfying K's bound: e.g.
>>
>> Test<?> <: [K:=Test<?>]Test<K> = Test<Test<?>> (false because
>> type-argument Test<?> does not contain ?)
>>
>> I think there's only one solution to this problem, which is the
>> following:
>>
>> K inferred as #1, where #1 is a captured type variable whose upper
>> bound
>> is Test<#1>. Let's see how this works w.r.t. to bound checking:
>>
>> #1 <; [K:=#1]Test<K> = Test<#1>
>> ub(#1) = Test<#1> <: Test<#1> (true!)
>>
>> This solution has all the required properties, in that (i) the
>> inferred
>> type does not contain reference to the type to be inferred (T) and
>> (ii)
>> the inferred type satisfy K's declared bound.
>>
>> As you can see, the right answer is not that trivial - moreover as
>> Alex
>> mentioned in the related JLS CR [3], an acceptable solution should be
>> able to deal with more complex cases like:
>>
>> class Foo {
>> <T extends List<U>, U extends List<T>> U foo() {return null;}
>> List<?> s = foo();
>> }
>>
>> Here we have two mutually referring recursive bounds (aaargh!). My
>> solution works by inferring #1 (extends List<#2>) for T and #2
>> (extends
>> List<#1>) for U.
>>
>> Final note:
>> Since my approach makes use of some synthetically generated
>> captured-type variables with recursive bounds, sometimes javac might
>> crash because of a problem in the toString routine for CapturedTypes
>> (infinite loop). A recent, separate diagnostic work, ease this problem
>> by truncating the infinite regression - what I'm saying is that,
>> wthout
>> proper diagnostic support, it won't be possible to support this fix as
>> it is. The problem is that each captured type in the javac world is
>> associated to an underlying wildcard type argument (the one that
>> originated the captured type). In this case it's obvious that,
>> given the
>> fact that the captured type variable I'm introducing is synthetic,
>> there's no such wildcard type argument - however I believe that
>> displaying a meaningful wildcard to the user could help him understand
>> what went wrong in case of errors - but one could always replace the
>> synthetic captured type's wildcard argument with a simpler type,
>> such as
>> Test<?> (instead of the more precise Test<? extends #1> which leads to
>> infinite recursion).
>>
>> Maurizio
>>
>> [1] http://cr.openjdk.java.net/~mcimadamore/6369605/<http://cr.openjdk.java.net/%7Emcimadamore/6369605/>
>> <http://cr.openjdk.java.net/%7Emcimadamore/6369605/>
>> [2] http://bugs.sun.com/view_bug.do?bug_id=4879776
>> [3] http://bugs.sun.com/view_bug.do?bug_id=6369608
>> [4] http://bugs.sun.com/view_bug.do?bug_id=6369605
>>
>>
>>
>>
>
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