Daniel Sun created GROOVY-12447:
-----------------------------------
Summary: 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
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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