Properly handle while loops in Tailrec

A while loop condition or body is not in tail position.

Author: allanrenucci

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

@@ -373,6 +373,12 @@ class TailRec extends MiniPhase {
             noTailTransform(tree.finalizer)
           )
 
+        case tree @ WhileDo(cond, body) =>
+          cpy.WhileDo(tree)(
+            noTailTransform(cond),
+            noTailTransform(body)
+          )
+
         case _: Alternative | _: Bind =>
           assert(false, "We should never have gotten inside a pattern")
           tree

tests/neg-tailcall/while-loops.scala

@@ -0,0 +1,16 @@
+import annotation.tailrec
+
+object WhileLoops {
+  def cond: Boolean = ???
+
+  @tailrec def rec1: Unit = { // error: tailrec not applicable
+    while (cond) {
+      rec1 // error: not in tail position
+    }
+  }
+
+  @tailrec def rec2: Boolean = { // error: tailrec not applicable
+    while (rec2) { } // error: not in tail position
+    true
+  }
+}