Potentially unnecessary athrow is emitted when return is inside a nested block

[biboudis]

The following snippet generates an athrow instruction if return is used inside a nested block.

class Test {
  def fun: Int = {
    var a = 10;
    { // throw is generated only when return is nested
      return a
    }
  }
}

This seems like e a backend issue and may not be related with how return expressions are translated (because NonLocalReturns phase has no effect).

[allanrenucci]

Here is the generated bytecode:

Compiled from "Test.scala"
public class Test {
  public Test();
    Code:
       0: aload_0
       1: invokespecial #9                  // Method java/lang/Object."<init>":()V
       4: return

  public int fun();
    Code:
       0: bipush        10
       2: istore_1
       3: iload_1
       4: ireturn
       5: athrow
}

[sjrd]

The generated code is now

public class Test {
  public Test();
    Code:
       0: aload_0
       1: invokespecial #9                  // Method java/lang/Object."<init>":()V
       4: return

  public int fun();
    Code:
       0: bipush        10
       2: istore_1
       3: iload_1
       4: ireturn
}

So this is fixed. It was probably fixed by #14890

Not adding a regression test because it's not an actual error and it most likely covered by the tests in the PR anyway.