Scott Murphy Heiberg created GROOVY-12754:
---------------------------------------------
Summary: 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
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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