Fix #196. Errorneus tail-optimization of inner DefDef's

[DarkDimius]

Unless a DefDef is a result of desugaring an exception handler it shouldn't be optimized by tailrec.

There's a potential improvement here: to detect that a DefDef
is a label def which itself is called only in tail position.

This is a requirement to implement #194

[DarkDimius]

@odersky please review. Also see comment in #196

[retronym]

Perhaps I'm misunderstanding this, but why would nested functions be ineligible for tail call optimization?

scala> class Tail { def foo = { def bar(x: Int): Int = if (x == 0) 0 else bar(x - 1); bar(10) } }
defined class Tail

scala> :javap -v -p Tail#bar$1
  private final int bar$1(int);
    descriptor: (I)I
    flags: ACC_PRIVATE, ACC_FINAL
    Code:
      stack=2, locals=2, args_size=2
         0: iload_1
         1: iconst_0
         2: if_icmpne     7
         5: iconst_0
         6: ireturn
         7: iload_1
         8: iconst_1
         9: isub
        10: istore_1
        11: goto          0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      14     0  this   LTail;
            0      14     1     x   I
      LineNumberTable:
        line 7: 0
      StackMapTable: number_of_entries = 2
        frame_type = 0 /* same */
        frame_type = 6 /* same */

[DarkDimius]

They are eligible themselves and in example that you are presenting, even after this commit the bar will be optimized. But the calls to outer function shouldn't be treated as available to tail call optimization, and this is the bug that this commit fixes.

eg for such code:

def foo: Int = {
  def bar = foo
  bar
  foo
}

The call to foo inside bar shouldn't get replaced with a call to a LabelDef.

[retronym]

Thanks for the clarification. Perhaps a test case is warranted, or a reference to an existing test that now will pass as a result of this.

[odersky]

LGTM