Potential options for different handling of defined and implicit inne…

…r methods

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

@@ -438,12 +438,15 @@ class TailRec extends MiniPhase {
 
         case tree: DefDef =>
           if (isMandatory)
-            // We cant tail recurse through nested definitions, so dont want to propagate to child nodes
-            // We dont want to fail if there is a call that would recurse (as this would be a non self recurse), so dont
-            // want to call noTailTransform
-            // We can however warn in this case, as its likely in this situation that someone would expect a tail
-            // recursion optimization and enabling this to optimise would be a simple case of inlining the inner method
-            new NestedTailRecAlerter(method, tree.symbol).traverse(tree)
+            if (tree.symbol.is(Synthetic))
+              noTailTransform(tree.rhs)
+            else
+              // We cant tail recurse through nested definitions, so dont want to propagate to child nodes
+              // We dont want to fail if there is a call that would recurse (as this would be a non self recurse), so dont
+              // want to call noTailTransform
+              // We can however warn in this case, as its likely in this situation that someone would expect a tail
+              // recursion optimization and enabling this to optimise would be a simple case of inlining the inner method
+              new NestedTailRecAlerter(method, tree.symbol).traverse(tree)
           tree
 
         case _: Super | _: This | _: Literal | _: TypeTree | _: TypeDef | EmptyTree =>

tests/neg/i20105.scala

@@ -1,5 +1,6 @@
+import scala.annotation.tailrec
 @tailrec
-def foo(): Unit =
+def foo(): Unit = // error
   def bar(): Unit =
     if (???)
       foo()

tests/neg/i5397.scala

@@ -16,8 +16,10 @@ object Test {
         rec3 // error: not in tail position
     })
 
-  @tailrec def rec4: Unit = {
-    def local = rec4 // error: not in tail position
+  // This is technically not breaching tail recursion as rec4 does not call itself, local does
+  // This instead fails due to having no tail recursion at all
+  @tailrec def rec4: Unit = { // error: no recursive calls
+    def local = rec4
   }
 
   @tailrec def rec5: Int = {

tests/warn/i20105.scala

@@ -1,3 +1,4 @@
+import scala.annotation.tailrec
 @tailrec
 def foo(): Unit =
   def bar(): Unit =