Question regarding exhaustiveness check of record patterns

[Remi Forax]

> From: "Tesla Ice Zhang" <ice1000kotlin at foxmail.com>
> To: "amber-dev" <amber-dev at openjdk.org>
> Sent: Wednesday, November 2, 2022 7:59:52 PM
> Subject: Question regarding exhaustiveness check of record patterns

> Hi,
> I am confused by a compile error generated by the following code:

> sealed interface I<T> {}
> record A<T>(T t) implements I<T> {}
> public static void main(I<String> args) {
> switch (args) {
> case A<String>(var x) -> System.out.println(x);
> }
> }

> Compiling the code with the following JDK:

> openjdk 19 2022-09-20
> OpenJDK Runtime Environment (build 19+37)
> OpenJDK 64-Bit Server VM (build 19+37, mixed mode, sharing)

> I get:

> .\JavaBug.java:5: error: the switch statement does not cover all possible input
> values
> switch (args) {
> ^
> Note: .\JavaBug.java uses preview features of Java SE 19.
> Note: Recompile with -Xlint:preview for details.
> 1 error

> What cases could be not covered? It looks like a bug to me.

It is a bug, i'm able to reproduce it. 

TeslaSwitchTest.java:6: error: the switch statement does not cover all possible input values 
switch (args) { 
^ 
Note: TeslaSwitchTest.java uses preview features of Java SE 20. 
Note: Recompile with -Xlint:preview for details. 
1 error 

As a workaround, using a type parameter works 

static <T> void main(I<T> args) { 
switch (args) { 
case A<T>(var x) -> System.out.println(x); 
} 
} 

> Also, I believe that showing the particular uncovered cases in the error
> messages would make very good developer experiences.

yes ! 

> Best regards,
> Tesla

Rémi 
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