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* SI-9170 More flexible SessionTestSom Snytt2015-03-031-0/+58
| | | | | | | | SessionTest session text can include line continuations and pasted text. Pasted script (which looks like a double prompt) probably doesn't work. This commit includes @retronym's SI-9170 one-liner.
* SI-9126 Missing .seqs causes problems with parallel GenXsRex Kerr2015-02-202-20/+44
| | | | | | | | Added `.seq` in two essential places so that a parallel collection passed as an argument won't mess up side-effecting sequential computations in `List.flatMap` and `GenericTraversableTemplate.transpose` (thanks to retronym for finding the danger spots). Tests that `.seq` is called by constructing a `GenSeq` whose `.seq` disagrees with anything non-`.seq` (idea & working implementation from retronym). Also updates the `.seq` test for `Vector#++` to use the new more efficient method.
* Merge pull request #4347 from retronym/ticket/8801Adriaan Moors2015-02-191-0/+21
|\ | | | | SI-8801 Another test for fixed exponential-time compilation
| * SI-8801 Another test for fixed exponential-time compilationJason Zaugg2015-02-201-0/+21
| | | | | | | | | | | | | | | | | | | | | | Turns out that SI-9157 was a duplicate of SI-8801. This commit adds Paul's test, whose compile time is now back in the troposphere. % time qscalac test/files/pos/t8801.scala real 0m1.294s user 0m3.978s sys 0m0.240s
* | Merge pull request #4340 from retronym/topic/infix-completionAdriaan Moors2015-02-196-0/+412
|\ \ | | | | | | SI-9153 More complete and stable results for completions
| * | Remove incorrect completions: implicits can't add type membersJason Zaugg2015-02-172-4/+2
| | | | | | | | | | | | | | | | | | | | | The checkfile of the tests added in the last commit offered a type member from `RichInt` in the completions for the type `Int`. However, only term members can be extension methods; type members cannot.
| * | SI-9153 More complete and stable results for completionsJason Zaugg2015-02-176-0/+414
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Three items of background are needed to understand this bug. 1. When typechecking an application like `qual.m({stats; expr})`, the argument is typechecked using the formal parameter type of `m` as the expected type. If this fails with a type error located within in `expr`, the typer instead re-typechecks under `ContextMode.ReTyping` without an expected type, and then searches for an implicit adaptation to enable `view(qual).m(args)`. Under this mode, `Typer#typed1` clears the type of incoming trees. 2. The presentation compiler performs targetted operations like type completions by: - typechecking the enclosing tree, registering all typechecker `Context`s created in the process (`registerContext`) - finding the smallest enclosing `Context` around the target position (`doLocateContext`) - Using this context to perform implicit search, which can contribute members to the completion. (`applicableViews` within `interactive.Global#typeMembers`) 3. When verifiying whether or not a candidate implicit is applicable as a view from `F => T`, implicit search typechecks a dummy call of the form `q"candiate(${Ident("<argument>").setType(typeOf[F])})". Now, picture yourself at the nexus of these three storms. In the enclosed test case, we search for completions at: x + 1.<caret> 1. Because the code is incomplete, the application of `Int#+` doesn't typecheck, and the typer also tries to adapt `x` to a method applicable to the re-typechecked argument. 2. This process registers a context with `retypechecking` set to true. (If multiple contexts at the same position are registered, the last one wins.) 3. Implicit search uses this context to typecheck `Predef.Ensuring(<argument>.setType(Int))`, but the argument is promptly stripped of its type and retypechecking fails as there is no definition named `<argument>` in scope. As such, we missed out on extension methods, like `ensuring` in the list of completions. This commit changes the presentation compiler to turn off retyping mode in the context before starting to work with it. (Are the other modes that might cause similar bugs?) Once I made that change, I noticed that the results the enclosed test was not stable. I tracked this down to the use of a `HashMap` to carry the applicable implicit views, together with the way that the presentation compiler removes duplicates. This commit switched to a `LinkedHashMap`.
* | | Merge pull request #4330 from Ichoran/issue/8917Adriaan Moors2015-02-191-0/+9
|\ \ \ | | | | | | | | SI-8917 collection.mutable.BitSet's &= operator doesn't clear end
| * | | SI-8917 collection.mutable.BitSet's &= operator doesn't clear endRex Kerr2015-02-131-0/+9
| |/ / | | | | | | | | | Made &= run to the end of its own bitset, ignoring the size of what it's &-ing with (which is exactly what you want for &=).
* | | Merge pull request #4319 from axel22/fix-si-7943Adriaan Moors2015-02-191-0/+19
|\ \ \ | | | | | | | | Fix SI-7943 -- make `TrieMap.getOrElseUpdate` atomic.
| * | | Fix SI-7943 -- make `TrieMap.getOrElseUpdate` atomic.Aleksandar Prokopec2015-02-181-0/+19
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Override `getOrElseUpdate` method in `TrieMap`. The signature and contract of this method corresponds closely to the `computeIfAbsent` from `java.util.concurrent.ConcurrentMap`. Eventually, `computeIfAbsent` should be added to `scala.collection.concurrent.Map`. Add tests. Review by @Ichoran
* | | | Fix the Scaladoc tests under Java 8Jason Zaugg2015-02-191-0/+8
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Partest seems to require a second copy of the `filters` find in the root of the Scaladoc tests. This file ignores JVM-specific console output, like everone's favourite nag: Java HotSpot(TM) 64-Bit Server VM warning: ignoring option MaxPermSize=256M; support was removed in 8.0 Not sure what changed to make this necessary, maybe a partest upgrade? The first failing build was on Feb 14: https://scala-webapps.epfl.ch/jenkins/view/2.N.x/job/scala-nightly-auxjvm-2.11.x/287/
* | | | Merge pull request #4329 from adriaanm/followup-4235Adriaan Moors2015-02-181-0/+29
|\ \ \ \ | | | | | | | | | | Simpler & Scala.js-friendly `ClassTag.unapply`
| * | | | Use if/then/else to avoid box(unbox(_))Adriaan Moors2015-02-181-0/+29
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* | | | | Merge pull request #4309 from som-snytt/issue/9116-bAdriaan Moors2015-02-182-3/+9
|\ \ \ \ \ | | | | | | | | | | | | SI-9116 Set.subsets has a param list
| * | | | | SI-9116 Set.subsets has a param listSom Snytt2015-02-092-3/+9
| | |/ / / | |/| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Now both of the overloaded variants have a parameter list. This seems to make type inference happier. Or it makes someone happier. The user is unaware whether `subsets()` takes a default arg. But happily, empty application still kicks in.
* | | | | Merge pull request #4341 from retronym/ticket/9157Adriaan Moors2015-02-181-0/+13
|\ \ \ \ \ | | |_|_|/ | |/| | | SI-9157 Avoid exponential blowup with chained type projections
| * | | | SI-9157 Avoid exponential blowup with chained type projectionsJason Zaugg2015-02-181-0/+13
| | |_|/ | |/| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Calling `findMember` in the enclosed test was calling into `NonClassTypeRef#relativeInfo` an exponentially-increasing number of times, with respect to the length of the chained type projections. The numbers of calls increased as: 26, 326, 3336, 33446, 334556. Can any pattern spotters in the crowd that can identify the sequence? (I can't.) Tracing the calls saw we were computing the same `memberType` repeatedly. This part of the method was not guarded by the cache. I have changed the method to use the standard idiom of using the current period for cache invalidation. The enclosed test now compiles promptly, rather than in geological time.
* | | | Merge pull request #4328 from adriaanm/rework-4230Adriaan Moors2015-02-171-0/+15
|\ \ \ \ | |/ / / |/| | | SI-9059 Random.alphanumeric is inefficient
| * | | SI-9059 Random.alphanumeric is inefficientDenton Cockburn2015-02-131-0/+15
| | |/ | |/| | | | | | | | | | | | | | | | | | | | | | Generate alphanumeric values by taking random element of string containing all alphanumerics. Instead of generating nextPrintableChar then filtering for alphanumerics, we can just take from a string containing all alphanumerics. This provides a significant performance improvement.
* | | Merge pull request #4296 from lrytz/t9105Adriaan Moors2015-02-137-27/+483
|\ \ \ | | | | | | | | Fixes and tests for InnerClass / EnclsoingMethod classfile attributes
| * | | Test java reflection on scala library / reflect / compiler jarsLukas Rytz2015-02-071-0/+65
| | | | | | | | | | | | | | | | | | | | | | | | | | | | Run a number of Java reflection operations on all classes in scala library / reflect / compiler. The test is based on a draft by Jason.
| * | | Fix InnerClass / EnclosingMethod for closures nested in value classesLukas Rytz2015-02-074-13/+86
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Members of value classes are moved over to the companion object early. This change ensures that closure classes nested in value classes appear that way to Java reflection. This commit also changes the EnclosingMethod attribute for classes (and anonymous functions) nested in anonymous function bodies. Before, the enclosing method was in some cases the function's apply method. Not always though: () => { class C ... val a = { class D ...} } The class C used to be nested in the function's apply method, but not D, because the value definition for a was lifted out of the apply. After this commit, we uniformly set the enclosing method of classes nested in function bodies to `null`. This is consistent with the source-level view of the code. Note that under delambdafy:method, closures never appear as enclosing classes (this didn't change in this commit).
| * | | Fix InnerClass/EnclosingMethod for trait impl and specialized classesLukas Rytz2015-02-072-2/+124
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Trait implementation classes and specialized classes are always considered top-level in terms of the InnerClass / EnclosingMethod attributes. These attributes describe source-level properties, and such classes are a compilation artifact. Note that the same is true for delambdafy:method closure classes (they are always top-level).
| * | | SI-9124 fix EnclosingMethod of classes nested in implOnly trait defsLukas Rytz2015-02-072-0/+48
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Private trait methods are not added to the generated interface, they end up only in the implementation class. For classes nested in such methods, the EnclosingMethod attribute was incorrect. Since the EnclosingMethod attribute expresses a source-level property, but the actual enclosing method does not exist in the bytecode, we set the enclosing method to null.
| * | | SI-9105 Fix EnclosingMethod for classes defined in lambdasLukas Rytz2015-02-075-19/+167
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | This change fixes both GenASM and GenBCode, except for the change to renaming in LamdaLift mentioned below. The reason for an inconsistent EnclosingMethod attribute was the symbol owner chain. Initially, closure class symbols don't exist, they are only created in UnCurry (delambdafy:inline). So walking the originalOwner of a definition does not yield closure classes. The commit also fixes uses of isAnonymousClass, isAnonymousFunction and isDelambdafyFunction in two ways: 1. by phase-travelling to an early phase. after flatten, the name includes the name of outer classes, so the properties may become accidentally true (they check for a substring in the name) 2. by ensuring that the (destructive) renames during LambdaLift don't make the above properties accidentally true. This was in fact the cause for SI-8900.
* | | | Merge pull request #4304 from adriaanm/rebase-4219Adriaan Moors2015-02-132-17/+16
|\ \ \ \ | |_|/ / |/| | | SI-8818 FreshName extractor forgives suffix
| * | | SI-8818 FreshName extractor forgives suffixSom Snytt2015-02-092-17/+16
| | | | | | | | | | | | | | | | | | | | | | | | The test is corrected (inverted) and the extractor is made more succinct. Succinctness isn't enforced by the test, but I checked it manually.
* | | | Merge pull request #4195 from lrytz/t8841Adriaan Moors2015-02-136-11/+52
|\ \ \ \ | |_|_|/ |/| | | SI-8841 report named arg / assignment ambiguity also in silent mode.
| * | | SI-8841 report named arg / assignment ambiguity also in silent mode.Lukas Rytz2014-12-126-11/+52
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | For local definitions (eg. in a block that is an argument of a method call), the type completer may have a silent context. A CyclicReference is then not thrown but transformed into a NormalTypeError. When deciding if 'x = e' is an assignment or a named arg, we need to report cyclic references, but not other type errors. In the above case, the cyclic reference was not reported. Also makes sure that warnings are printed after typing argument expressions.
* | | | Merge pull request #4263 from lrytz/t9097Adriaan Moors2015-02-091-0/+34
|\ \ \ \ | | | | | | | | | | SI-9097 Remove spurious warning about conflicting filenames
| * | | | SI-9097 Consolidate testSom Snytt2015-01-203-12/+10
| | | | | | | | | | | | | | | | | | | | `pos` test is subsumed by `run`.
| * | | | SI-9097 Remove spurious warning about conflicting filenamesLukas Rytz2015-01-203-0/+36
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | When using delambdafy:method, closure classes are generated late. The class is added to a map and integrated into the PackageDef in transformStats. When declaring a package object, there are potentially multiple PackageDefs for the same package. In this case, the closure class was added to all of them. As a result, GenASM / GenBCode would run multiple times on the closure class. In GenBCode this would trigger a warning about conflicting filenames.
* | | | | Merge pull request #4280 from kanielc/SI-9095Adriaan Moors2015-02-092-0/+50
|\ \ \ \ \ | | | | | | | | | | | | SI-9095 Memory leak in LinkedHasMap and LinkedHashSet
| * | | | | SI-9095 Memory leak in LinkedHasMap and LinkedHashSetDenton Cockburn2015-01-312-0/+50
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | The clear() method in scala.collection.mutable.LinkedHashSet and scala.collection.mutable.LinkedHashMap does not set lastEntry to null. In result it holds a reference to an object that was removed from the collection.
* | | | | | Merge pull request #4209 from kanielc/SI-8988Adriaan Moors2015-02-091-0/+37
|\ \ \ \ \ \ | |_|_|_|/ / |/| | | | | SI-8988 Escaping character in StringLike.split(c) is slow
| * | | | | SI-8988 Escaping character in StringLike.split(c) prevents usage of ↵Denton Cockburn2015-01-091-0/+37
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | optimized String.split code path Escaping a char when calling split is slow. We end up compiling a Pattern to simply match a character literal. Instead, we just use an loop with indexOf to construct our resulting Array. Current speed up over old behaviour is about 12-1
* | | | | | Merge pull request #4288 from adriaanm/retronym-ticket/9041Adriaan Moors2015-02-092-0/+21
|\ \ \ \ \ \ | | | | | | | | | | | | | | SI-9041 Avoid unreported type error with overloading, implicits
| * | | | | | SI-9041 Avoid unreported type error with overloading, implicitsJason Zaugg2015-02-022-0/+21
| | |/ / / / | |/| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | If `qual.foo(args)` fails to typecheck, we fall back to `ImplicitView(qual).foo(args)`. However, if the original type error stemmed from an overload ambiguity, the tree `Select(qual, 'foo')` holds onto an error symbol. The fall back attempt just returns an `Apply` tree containing the erroneous qualifier, as it does not want to issue cascading type errors. This commit replaces the error symbol with a `NoSymbol`, which triggers the second try typechecking to perform overload resolution again. A more principled fix might be to more pervasively duplicate trees before mutating their types and symbols, that this is beyond the scope of this bug fix.
* | | | | | Merge pull request #4274 from retronym/ticket/9123Adriaan Moors2015-02-092-0/+11
|\ \ \ \ \ \ | |_|_|_|_|/ |/| | | | | SI-9123 More coherent trees with patmat, dependent types
| * | | | | SI-9123 More coherent trees with patmat, dependent typesJason Zaugg2015-01-292-0/+11
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | The pattern matcher needs to substitute references to bound variables with references to either a) synthetic temporary vals, or to b) selections. The latter occurs under -optimize to avoid to be frugal with local variable slots. For instance: ``` def test(s: Some[String]) = s match { case Some(elem) => elem.length } ``` Is translated to: ``` def test(s: Some[String]): Int = { case <synthetic> val x1: Some[String] = s; case4(){ if (x1.ne(null)) matchEnd3(x1.x.length()) else case5() }; case5(){ matchEnd3(throw new MatchError(x1)) }; matchEnd3(x: Int){ x } } ``` However, for a long time this translation failed to consider references to the binder in types. #4122 tried to address this by either using standard substitution facilities where available (references to temp vals), and by expanding the patmat's home grown substitution to handle the more complex case of referencing a selection. However, this left the tree in an incoherent state; while it patched up the `.tpe` field of `Tree`s, it failed to modify the info of `Symbol`-s. This led to a crash in the later uncurry phase under `-Ydelambdafy:method`. This commit modifies the info of such symbols to get rid of stray refeferences to the pattern binder symbols.
* | | | | | Merge pull request #4295 from retronym/ticket/9135Grzegorz Kossakowski2015-02-051-0/+16
|\ \ \ \ \ \ | | | | | | | | | | | | | | SI-9135 Fix NPE, a regression in the pattern matcher
| * | | | | | SI-9135 Fix NPE, a regression in the pattern matcherJason Zaugg2015-02-051-0/+16
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | The community build discovered that #4252 introduced the possibility for a NullPointerException. The tree with a null type was a synthetic `Apply(<<matchEnd>>)` created by the pattern matcher. This commit adds a null check.
* | | | | | | Merge pull request #4251 from retronym/ticket/6502Jason Zaugg2015-02-052-28/+50
|\ \ \ \ \ \ \ | | | | | | | | | | | | | | | | SI-6502 More robust REPL :require
| * | | | | | | SI-6502 Convert test to assertsSom Snytt2015-01-202-29/+25
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | This saves a check file in the crowded test directory.
| * | | | | | | SI-6502 More robust REPL :requireJason Zaugg2015-01-161-0/+26
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | - handle missing files gracefully (rather than NPE) - read the class name with ASM, rather than with a dummy classloader. The dummy classloader is prone to throwing `LinkageError`s, as reported in the comments of SI-6502. Manual test of the original report: ``` % qscala Welcome to Scala version 2.11.5-20150115-183424-155dbf3fdf (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_25). Type in expressions to have them evaluated. Type :help for more information. scala> :require does/not/exist Cannot read: does/not/exist scala> classOf[org.junit.Test] <console>:8: error: object junit is not a member of package org classOf[org.junit.Test] ^ scala> :require /Users/jason/.m2/repository/junit/junit/4.11/junit-4.11.jar Added '/Users/jason/.m2/repository/junit/junit/4.11/junit-4.11.jar' to classpath. scala> classOf[org.junit.Test] res1: Class[org.junit.Test] = interface org.junit.Test ``` I have commited an automated test that is a minimization of this one.
* | | | | | | | Merge pull request #4248 from retronym/ticket/9086Jason Zaugg2015-02-051-0/+8
|\ \ \ \ \ \ \ \ | |_|/ / / / / / |/| | | | | | | SI-9086 Fix regression in implicit search
| * | | | | | | SI-9086 Fix regression in implicit searchJason Zaugg2015-01-291-0/+8
| | |/ / / / / | |/| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Implicit search declines to force the info of candidate implicits that either a) are defined beyond the position of the implicit search site, or b) enclose the implicit search site. The second criterion used to prevent consideration of `O` in the super constructor call: implicit object O extends C( { implicitly[X] }) However, after https://github.com/scala/scala/pull/4043, the block containing the implicit search is typechecked in a context owned by a local dummy symbol rather than by `O`. (The dummy and `O` share an owner.) This led to `O` being considered as a candidate for this implicit search. This search is undertaken during completion of the info of `O`, which leads to it being excluded on account of the LOCKED flag. Unfortunately, this also excludes it from use in implicit search sites subsequent to `O`, as `ImplicitInfo` caches `isCyclicOrErroneous`. This commit adjusts the position of the local dummy to be identical to that of the object. This serves to exclude `O` as a candidate during the super call on account of criterion a).
* | | | | | | Merge pull request #4133 from som-snytt/issue/8976Ichoran2015-02-044-6/+113
|\ \ \ \ \ \ \ | | | | | | | | | | | | | | | | SI-8976 MutableList.tail.iterator.size is length
| * | | | | | | SI-8976 MutableList.tail.iterator.size is lengthSom Snytt2014-12-064-6/+113
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | The previous behavior was to iterate over the mutated list of arbitrary length. The previous iteration of the iterator would also iterate the terminal element of the list without halting. This is fixed by capping the length of iterator. That is OK because mutating the list by adding to it during iteration is not recommended. For good measure, the exhausted iterator does not hold a reference to any remaining tail. A test is added that will no doubt be superseded by the QCC tests. (Quasi-Comprehensive Collections.) The test just checks that the extra tail is not strongly reachable from the iterator. If the garbage collector happens to kick in and determine that the object is weakly reachable, then the check terminates early.