expression type A is a subtype of pattern type B diagnostics

[Liam Miller-Cushon]

Hi,

I had a question about a change that was made to instanceof patterns as
part of JEP 440.

javac emits diagnostics for instanceof patterns where the expression type
is a subtype of the pattern type in JDK 20, but the diagnostics were
removed in 21 as part of the implementation of JEP 440.

The spec diff
<https://cr.openjdk.org/~gbierman/jep440%2B441/jep440%2B441-20230509/specs/patterns-switch-record-patterns-jls.html#jls-15.20>
for those changes removed language from 15.20 about

If the type of the RelationalExpression is a subtype of the type of the
> Pattern, then a compile-time error occurs.


The implementation also updated
test/langtools/tools/javac/patterns/NoSubtypeCheck.java to test for those
diagnostics when --release 20 is passed.

So all of this seems deliberate and working as intended, I was just curious
what the rationale for the change was. Does anyone remember the history or
have pointers to discussions about it on list?

public class Z {
  class A {}
  class B extends A {}
  void f(B b) {
    if (b instanceof A a) {}
  }
}

Z.java:5: error: expression type Z.B is a subtype of pattern type Z.A
    if (b instanceof A a) {}
          ^
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