Cast pattern executes even when operand subpattern is refuted

Example taken from tests/language/patterns/exhaustiveness/nullable_cast_test.dart in the SDK:

import 'dart:async';

exhaustiveNonNullableFutureOrTypeVariable2<T extends Object>(FutureOr<T?> o) =>
    switch (o) {
      FutureOr<T>() as FutureOr<T> => 0,
    };

main() {
  exhaustiveNonNullableFutureOrTypeVariable2<int>(
      Future<int?>.value(null));
}

This code is currently expected to throw. I find it unintuitive that the cast in a cast pattern will execute even when the operand is refuted. Can we change the semantics so that the cast only occurs if the pattern match on the operand succeeds?

Start with the example in tests/language/patterns/exhaustiveness/nullable_cast_test.dart, especially the referenced cast-pattern case. Run the test to confirm the current throw, then read the surrounding test and the linked language issue context to identify the specified cast-pattern semantics. Done means the behavior and corresponding language specification tests reflect whether the operand subpattern succeeds before casting.