[jira] [Commented] (GROOVY-12447) STC: treat a complete native sealed type-case set as an exhaustive switch expression

["ASF GitHub Bot (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/GROOVY-12447?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18126044#comment-18126044 ] 

ASF GitHub Bot commented on GROOVY-12447:
-----------------------------------------

daniellansun opened a new pull request, #2975:
URL: https://github.com/apache/groovy/pull/2975

   https://issues.apache.org/jira/browse/GROOVY-12447




> STC: treat a complete native sealed type-case set as an exhaustive switch expression
> ------------------------------------------------------------------------------------
>
>                 Key: GROOVY-12447
>                 URL: https://issues.apache.org/jira/browse/GROOVY-12447
>             Project: Groovy
>          Issue Type: Improvement
>            Reporter: Daniel Sun
>            Priority: Major
>
> h2. Context
> GROOVY-12255 compiles switch expressions as first-class AST ([JEP 361|https://openjdk.org/jeps/361]). Under {{@TypeChecked}} and {{@CompileStatic}}, a switch expression is exhaustive when it has a {{default}} arm, or when the selector is an enum and every constant is listed.
> Native sealed types (GROOVY-10148, GROOVY-10233, [JEP 409|https://openjdk.org/jeps/409]) are a closed hierarchy when the target bytecode is JDK 17+ and the sealed mode is native. Their permitted subclasses are known at compile time, the same way enum constants are.
> Groovy already matches {{case Circle}} with {{isCase}} ({{Class.isInstance}}). That type-case form is what this issue uses.
> h2. Problem
> STC rejects a switch expression that names every permitted subclass of a native sealed selector:
> {code:groovy}
> sealed interface Shape permits Circle, Square {}
> final class Circle implements Shape {}
> final class Square implements Shape {}
> String name(Shape s) {
>     return switch (s) {
>         case Circle -> 'c'
>         case Square -> 's'
>     }
> }
> {code}
> The error is {{the switch expression does not cover all possible input values}}. The same program is legal in Java with type patterns ([JEP 441|https://openjdk.org/jeps/441]). Enums already receive this treatment in Groovy.
> h2. Contract
> A switch expression with no {{default}} is exhaustive when the selector type is native sealed and every remaining inhabitant is covered by a {{case Type}} label.
> Inhabitants of the selector are its permitted subclasses, plus the selector type itself when that type is a concrete class.
> A type-case label that names a member of the hierarchy removes remaining inhabitants that are that type or a subtype of it. {{case Circle, Square}} is two labels (the parser emits one {{CaseStatement}} per value). {{case Rect}} covers nested {{Square}} and {{Triangle}}. {{case Square}} and {{case Triangle}} leave {{Rect}} remaining.
> A label outside the hierarchy, including {{case Object}}, is ordinary {{isCase}} and leaves the remaining set unchanged.
> Native sealed is detected as follows:
> * source: {{SealedASTTransformation.wouldBeNativeSealed}} (JDK 17+ target bytecode and native mode), independent of transform order
> * class file: a non-empty permitted-subclass list (the {{PermittedSubclasses}} attribute). Emulated sealed class files keep a runtime {{@Sealed}} and leave that list empty.
> Duplicate permitted-subclass labels reuse the GROOVY-12289 / GROOVY-12406 check. The key is the pair (selector type, binary name), so {{case Circle}} and {{case 'Circle'}} are distinct. That check already runs for switch expressions and switch statements.
> ||Selector and labels||Today||This change||
> |native sealed interface, every permitted type|error|exhaustive|
> |one permitted type omitted|error|error|
> |emulated sealed, every permitted type|error|error|
> |concrete {{sealed class}}, subclasses only|error|error|
> |{{case Shape}} on a {{Shape}} selector|error|exhaustive|
> |{{case Object}} only|error|error|
> |{{case Circle}} and {{case Rect}} ({{Rect}} permits {{Square}}, {{Triangle}})|error|exhaustive|
> |{{case Circle}}, {{case Square}}, {{case Triangle}} ({{Rect}} still permitted)|error|error|
> |native sealed type from a class file, every permitted type|error|exhaustive|
> |switch statement, partial type cases|compiles|compiles|
> |enum, every constant (GROOVY-12255)|exhaustive|exhaustive|
> |duplicate {{case Circle}}|compiles|error: {{Duplicate case label: Circle}}|
> A concrete sealed class is itself an inhabitant:
> {code:groovy}
> sealed class Shape permits Circle, Square {}
> final class Circle extends Shape {}
> final class Square extends Shape {}
> String name(Shape s) {
>     return switch (s) {
>         case Circle -> 'c'
>         case Square -> 's'
>     }
> }
> {code}
> STC still reports {{the switch expression does not cover all possible input values}}, because {{new Shape()}} is legal. {{case Shape}} covers the whole hierarchy.
> Switch statements may be partial. Emulated sealed ({{@SealedOptions(mode = SealedMode.EMULATE)}}) still needs a {{default}} arm. Dynamic Groovy still throws {{IllegalStateException}} for an unmatched selector. Null stays outside this check, matching enum exhaustiveness.
> h2. Touch points
> * {{org.codehaus.groovy.transform.stc.StaticTypeCheckingVisitor}} — {{coversAllPermittedSubclasses}}, {{nativeSealedClosed}}, {{permittedSubclassLabelKey}}
> * {{src/test/groovy/groovy/transform/stc/SwitchExpressionSTCTest.groovy}}
> h2. Scope
> This issue covers STC exhaustiveness of native sealed type-case labels, and duplicate detection of those labels. GEP-19 pattern matching, flow typing of the matched arm (GROOVY-12300), and bytecode dispatch for type cases are separate work.



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