Notice if deferred impl selects us

Author: som-snytt

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3135,6 +3135,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         val rhs = implicitArgTree(target, cdef.span,
             where = i"inferring the implementation of the deferred ${dcl.showLocated}"
           )(using searchCtx)
+        val resolvedHere =
+          rhs.tpe match
+          case tp: NamedType => (tp.prefix.typeSymbol eq cls) && tp.name == mbr.name && !tp.typeSymbol.is(Method)
+          case _             => false
+        if resolvedHere then failFor(mbr, "the result is self-recursive")
 
         val impl = dcl.copy(cls,
           flags = dcl.flags &~ (HasDefault | Deferred) | Final | Override,

tests/neg/i22589b.scala

@@ -0,0 +1,16 @@
+
+import scala.compiletime.deferred
+
+trait MyCodec[E]
+
+object auto:
+  trait CompanionEssentials[E]:
+    given myc: MyCodec[E] = deferred
+
+import auto.CompanionEssentials
+
+case class Person(name: String, age: Int)
+object Person extends CompanionEssentials[Person]: // error
+  given String = "hw"
+  given myc(using String): MyCodec[Person] = new MyCodec[Person] {}
+  override def toString = ""

tests/pos/i22589.scala

@@ -0,0 +1,17 @@
+
+import scala.compiletime.deferred
+
+trait MyCodec[E]
+
+object auto:
+  trait CompanionEssentials[E]:
+    //given [E] => MyCodec[E] = deferred
+    given MyCodec[E] = deferred
+
+import auto.CompanionEssentials
+
+case class Person(name: String, age: Int)
+object Person extends CompanionEssentials[Person]:
+  //given something: [E] => MyCodec[E] = new MyCodec[E] {}
+  given something: MyCodec[Person] = new MyCodec[Person] {}
+  override def toString = ""