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https://issues.apache.org/jira/browse/GROOVY-12562?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18135271#comment-18135271
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ASF GitHub Bot commented on GROOVY-12562:
-----------------------------------------
testlens-app[bot] commented on PR #3091:
URL: https://github.com/apache/groovy/pull/3091#issuecomment-6527114507
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> STC: instanceof narrows a variable where the check does not guard the code
> --------------------------------------------------------------------------
>
> Key: GROOVY-12562
> URL: https://issues.apache.org/jira/browse/GROOVY-12562
> Project: Groovy
> Issue Type: Bug
> Components: Static Type Checker
> Affects Versions: 4.0.33, 5.0.8, 6.0.0
> Reporter: Scott Murphy Heiberg
> Priority: Major
>
> The type checker narrows a variable after any {{instanceof}} check on it, for
> the rest of the expression or statement, wherever the check is. Under
> {{@CompileStatic}} the program then fails with a ClassCastException:
> {code:groovy}
> @groovy.transform.CompileStatic
> String f(CharSequence cs) { 'x' + (cs instanceof String) + cs.length() }
> f(new StringBuilder('ab')) // ClassCastException: StringBuilder cannot be
> cast to String (5.0.8, 6.0.0)
> {code}
> The same holds for {{foo(cs instanceof String, cs.length())}}, {{[cs
> instanceof String, cs]}} and {{(cs instanceof String) == cs.isEmpty()}}. A
> comparison with {{false}} narrows the wrong branch, and the condition of a
> loop narrows after the loop:
> {code:groovy}
> @groovy.transform.CompileStatic
> int h(Object x) { if ((x instanceof String) == false) { return x.length() };
> 0 }
> h(42) // ClassCastException: Integer cannot be cast to String
> @groovy.transform.CompileStatic
> int g(Object x) { while (x instanceof String) { break }; x.length() }
> g(42) // ClassCastException
> {code}
> The last two fail in 4.0.33, 5.0.8 and 6.0.0, the first in 5.0.8 and 6.0.0.
> Expected: a check narrows only where its outcome guards what follows (the
> condition of an if, while, for, assert or conditional expression, and
> operands of &&, || and ! there); {{(x instanceof T) == true}} narrows like
> the check and {{== false}} like its negation; a loop condition narrows inside
> the loop only.
> Behaviour change under @TypeChecked and @CompileStatic: an instanceof check
> now narrows the variable only where its outcome guards the code (conditions,
> operands of &&, || and !, comparisons with a boolean literal, boolean casts,
> assignments in a condition), and the condition of a loop narrows inside the
> loop only. Code that relied on the old narrowing compiled before and is now a
> compile error, for example
> {code:groovy}
> @CompileStatic
> def f(Object x) { '' + (x instanceof String) + x.length() }
> // before: compiled; f(new StringBuilder('ab')) threw ClassCastException
> // now: [Static type checking] - Cannot find matching method
> java.lang.Object#length()
> {code}
> and likewise a call of {{x.length()}} after a {{while (x instanceof String)}}
> loop. A check that is a statement on its own did not narrow later statements
> before and does not now; switch/case and catch are unaffected. The PR adds a
> COMPATIBILITY.md entry.
> PR: https://github.com/apache/groovy/pull/3091
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