Fix #5397: Visit local methods and values in Tailrec

If a method is annotated with @tailrec we now report invalid recursive
call within local method. This means we now visit by-name arguments,
closures and lifted `try`s

Author: allanrenucci

compiler/src/dotty/tools/dotc/transform/TailRec.scala

@@ -10,6 +10,7 @@ import dotty.tools.dotc.core.Flags._
 import dotty.tools.dotc.core.NameKinds.{TailLabelName, TailLocalName, TailTempName}
 import dotty.tools.dotc.core.StdNames.nme
 import dotty.tools.dotc.core.Symbols._
+import dotty.tools.dotc.transform.SymUtils._
 import dotty.tools.dotc.reporting.diagnostic.messages.TailrecNotApplicable
 import dotty.tools.dotc.transform.MegaPhase.MiniPhase
 
@@ -407,8 +408,11 @@ class TailRec extends MiniPhase {
           assert(false, "We should never have gotten inside a pattern")
           tree
 
-        case _: ValDef | _: DefDef | _: Super | _: This |
-             _: Literal | _: TypeTree | _: TypeDef | EmptyTree =>
+        case tree: ValOrDefDef =>
+          if (isMandatory) noTailTransform(tree.rhs)
+          tree
+
+        case _: Super | _: This | _: Literal | _: TypeTree | _: TypeDef | EmptyTree =>
           tree
 
         case Labeled(bind, expr) =>

tests/neg-tailcall/t1672b.scala

@@ -43,12 +43,12 @@ object Test1772B {
 
   // the `liftedTree` local method will prevent a tail call here.
   @tailrec
-  def bar(i : Int) : Int = { // error: TailRec optimisation not applicable
+  def bar(i : Int) : Int = {
     if (i == 0) 0
     else 1 + (try {
       throw new RuntimeException
     } catch {
-      case _: Throwable => bar(i - 1) // old error: cannot rewrite recursive call
+      case _: Throwable => bar(i - 1) // error: it is not in tail position
     })
   }
 }

tests/neg/i5397.scala

@@ -0,0 +1,41 @@
+import annotation.tailrec
+
+object Test {
+  def foo(x: => Int) = 1
+  def bar(x: Int => Int) = 1
+
+  @tailrec def rec1: Int =
+    foo(rec1) // error not in tail position
+
+  @tailrec def rec2: Int =
+    bar(_ => rec2) // error not in tail position
+
+  @tailrec def rec3: Int =
+    1 + (try ??? catch {
+      case _: Throwable =>
+        rec3 // error not in tail position
+    })
+
+  @tailrec def rec4: Unit = {
+    def local = rec4 // error not in tail position
+  }
+
+  @tailrec def rec5: Int = {
+    val x = {
+      rec5 // error not in tail position
+      1
+    }
+    x
+  }
+}
+
+object Test4649 {
+  @tailrec
+  def lazyFilter[E](s: Stream[E], p: E => Boolean): Stream[E] = s match {
+    case h #:: t =>
+      if (p(h)) h #:: lazyFilter(t, p) // error not in tail position
+      else lazyFilter(t, p)
+    case _ =>
+      Stream.empty
+  }
+}

tests/pos/ErasureAnd.scala

@@ -1,6 +1,4 @@
-import scala.annotation.tailrec
 trait A { self: B =>
-  @tailrec
   private def foo(arg1: Int, arg2: Int): Int = {
     def bar = this.foo(arg1, arg2)
     foo(arg1, arg2)