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https://issues.apache.org/jira/browse/GROOVY-12754?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18141666#comment-18141666
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ASF GitHub Bot commented on GROOVY-12754:
-----------------------------------------
codeconsole opened a new pull request, #3285:
URL: https://github.com/apache/groovy/pull/3285
https://issues.apache.org/jira/browse/GROOVY-12754
`compatibleConnection` checked an upper-bounded wildcard argument against
the bound of a method's type parameter the wrong way round, asking whether the
bound fits within `? extends B`. So static type checking rejected calls that
javac accepts:
```groovy
static <T extends Number> T first(List<T> list) { list.get(0) }
static <T extends CharSequence> int len(Class<T> type) { 0 }
static Object a(List<? extends Integer> ints) { first(ints) }
static int b(Class<? extends String> type) { len(type) }
```
And it accepted one that javac rejects, `narrow(List<? extends Number>)` for
`<T extends Integer> T narrow(List<T>)`, where the statically compiled call
then returned `1` for `1.5`.
The capture of `? extends B` is a subtype of `B` (JLS 5.1.10), so the
wildcard is now checked as a type argument `B` is, by testing `? super B`
against the bound. An unbounded wildcard, one bounded by `Object` or by a type
variable, and a lower-bounded one keep the existing path. Code like `narrow`
above, which compiled before, is now rejected, as javac rejects it.
`GenericsJavaCompatibilityTest` gets a positive and a negative case checked
against javac. `GenericsSTCTest`, and so `GenericsStaticCompileTest`, gets the
Groovy forms, including a `Class<? extends ArrayList>` for `<T extends
List<Integer>>`. All of them fail without the change.
Please also apply this to GROOVY_6_0_X, where it cherry-picks cleanly.
GROOVY-12755 depends on it.
> STC: an upper-bounded wildcard argument is checked against a type parameter's
> bound the wrong way round
> -------------------------------------------------------------------------------------------------------
>
> Key: GROOVY-12754
> URL: https://issues.apache.org/jira/browse/GROOVY-12754
> Project: Groovy
> Issue Type: Bug
> Components: Static Type Checker
> Affects Versions: 6.0.0
> Reporter: Scott Murphy Heiberg
> Priority: Major
>
> Static type checking rejects a call that passes an upper-bounded wildcard,
> such as {{List<? extends Integer>}}, to a generic method whose type parameter
> is bounded by a supertype of the wildcard's bound. It accepts the call when
> the wildcard's bound is wider than the type parameter's. javac does the
> opposite in both cases.
> {code:groovy}
> @groovy.transform.CompileStatic
> class C {
> static <T extends Number> T first(List<T> list) { list.get(0) }
> static <T extends CharSequence> int len(Class<T> type) { 0 }
> static <T extends Integer> T narrow(List<T> list) { list.get(0) }
> static Object a(List<? extends Integer> ints) { first(ints) } //
> rejected, javac accepts
> static int b(Class<? extends String> type) { len(type) } //
> rejected, javac accepts
> static Object c(List<? extends Number> nums) { narrow(nums) } //
> accepted, javac rejects
> }
> {code}
> {noformat}
> [Static type checking] - Cannot call <T extends java.lang.Number>
> C#first(java.util.List<T>) with arguments [java.util.List<? extends
> java.lang.Integer>]
> [Static type checking] - Cannot call <T extends java.lang.CharSequence>
> C#len(java.lang.Class<T>) with arguments [java.lang.Class<? extends
> java.lang.String>]
> {noformat}
> The capture of {{? extends B}} is a subtype of {{B}} (JLS 5.1.10), so it is
> within the bound of {{T}} exactly when {{B}} is. javac reports "inference
> variable T has incompatible bounds" for {{c}}. Groovy compiles {{c}}, and
> {{c([1.5d])}} returns {{1}}.
> {{StaticTypeCheckingSupport#compatibleConnection}} tests a wildcard witness
> with {{connection.isCompatibleWith(bound)}}, which asks whether the type
> parameter's bound fits within {{? extends B}}. For a type argument {{B}}, it
> tests {{? super B}} against the bound. An upper-bounded wildcard should take
> the same test.
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