Paul King created GROOVY-12478:
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Summary: yield before the last statement of an arrow-switch arm is
ignored when the switch is compiled as a statement
Key: GROOVY-12478
URL: https://issues.apache.org/jira/browse/GROOVY-12478
Project: Groovy
Issue Type: Bug
Affects Versions: 6.0.0
Reporter: Paul King
Assignee: Paul King
Fix For: 6.0.1
A {{yield}} that is not the last statement of an arrow-switch block arm is
silently ignored when the switch ends up classified as a statement: execution
falls through to the rest of the arm instead of completing the switch.
{code:groovy}
def pick(boolean flag) {
switch (1) {
case 1 -> {
if (flag) yield 'first'
yield 'second'
}
}
}
assert pick(true) == 'first' // fails on 6.0.0: 'second'
{code}
The same thing happens when the switch's value is discarded (the code after the
conditional {{yield}} still runs), and a {{yield}} inside a loop within the arm
no longer compiles:
{code:groovy}
def pick(List xs) {
switch (1) {
case 1 -> {
for (x in xs) { if (x > 2) yield "big $x" }
yield 'none'
}
}
}
// 6.0.0: "the yield statement is only allowed inside a switch expression"
{code}
||Case||5.1.2||6.0.0-RC-1||6.0.0-RC-3 / 6.0.0||
|Trailing switch, {{pick(true)}}|'first'|'first'|*'second'*|
|Value discarded, conditional {{yield}}|arm exits|arm exits|*falls through*|
|{{yield}} inside a loop in the arm|works|works|*compile error*|
|{{return switch (...)}}|'first'|'first'|'first'|
Behaviour is the same in dynamic Groovy and under {{@CompileStatic}}.
*Cause:* since GROOVY-12408 a trailing switch is a statement, and the parser
rewrites statement-position arrow switches via
{{AstBuilder.armCodeAsStatement}}, which replaces each {{yield x}} with a plain
expression statement {{x}}. Only the last value of the arm reaches
{{ReturnAdder}}, so a final {{yield}} still appears to work, but any earlier
{{yield}} loses its jump. The rewrite does not descend into loops, so a nested
{{yield}} survives as a {{YieldStatement}} and is then rejected.
*Expected:* {{yield}} keeps its early-exit meaning wherever the switch's
position puts it. GROOVY-12408 was meant to change only how a trailing switch
is encoded, and 4.x/5.x code had to use {{yield}} in block arms of exactly
these switches. In return position, a {{yield}} should behave like {{return}}.
Elsewhere it should evaluate its expression and exit the switch.
*Suggested fix:* keep the {{YieldStatement}}s instead of flattening them. Let
{{ReturnAdder}} turn the yields belonging to a return-position switch into
returns, searching the whole arm including loops but stopping at nested switch
expressions and closures. Compile any remaining {{yield}} in a switch statement
as "evaluate, discard, jump to the switch's end label", which also makes the
loop case work.
*Workaround:* write {{return switch (...) \{ ... \}}} so the switch stays an
expression, or restructure the arm so its only {{yield}} is the last statement
(e.g. if/else with a {{yield}} in each branch).
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