Fail if deferred given impl is recursive

Author: som-snytt

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

@@ -3277,7 +3277,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           val superCls = cls.superClass
           superCls.exists && superCls.asClass.baseClasses.contains(m.symbol.owner)
 
-        def givenImpl(mbr: TermRef): ValDef =
+        def givenImpl(mbr: TermRef): ValDef | EmptyTree.type =
           val dcl = mbr.symbol
           val target = dcl.info.asSeenFrom(cls.thisType, dcl.owner)
           val constr = cls.primaryConstructor
@@ -3315,7 +3315,16 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 cpy.Select(id)(This(cls), id.name)
               case _ =>
                 super.transform(tree)
-          ValDef(impl, anchorParams.transform(rhs)).withSpan(impl.span.endPos)
+
+          rhs.tpe match
+          case tp: NamedType if tp.prefix.typeSymbol == cls && tp.name == mbr.name && !tp.typeSymbol.is(Method) =>
+            report.error(
+              em"""Inferred implementation of the deferred ${dcl.showLocated} is self-recursive.
+                  |An implementing given needs to be written explicitly.""",
+              cdef.srcPos)
+            EmptyTree
+          case _ =>
+            ValDef(impl, anchorParams.transform(rhs)).withSpan(impl.span.endPos)
         end givenImpl
 
         val givenImpls =

tests/neg/i22589.check

@@ -0,0 +1,7 @@
+-- Error: tests/neg/i22589.scala:15:7 ----------------------------------------------------------------------------------
+15 |object Person extends CompanionEssentials[Person]: // error
+   |^
+   |Inferred implementation of the deferred given instance given_MyCodec_E in trait CompanionEssentials is self-recursive.
+   |An implementing given needs to be written explicitly.
+16 |  //override final lazy given given_MyCodec_E: MyCodec[Person] = Person.given_MyCodec_E
+17 |  override def toString = ""

tests/neg/i22589.scala

@@ -0,0 +1,17 @@
+
+//> using options -Wsafe-init -Ysafe-init-global
+
+import scala.compiletime.deferred
+
+trait MyCodec[E]
+
+object auto:
+  trait CompanionEssentials[E]:
+    given MyCodec[E] = deferred
+
+import auto.CompanionEssentials
+
+case class Person(name: String, age: Int)
+object Person extends CompanionEssentials[Person]: // error
+  //override final lazy given given_MyCodec_E: MyCodec[Person] = Person.given_MyCodec_E
+  override def toString = ""

tests/neg/i22589b.check

@@ -0,0 +1,8 @@
+-- Error: tests/neg/i22589b.scala:13:7 ---------------------------------------------------------------------------------
+13 |object Person extends CompanionEssentials[Person]: // error
+   |^
+   |Inferred implementation of the deferred given instance myc in trait CompanionEssentials is self-recursive.
+   |An implementing given needs to be written explicitly.
+14 |  given String = "hw"
+15 |  given myc(using String): MyCodec[Person] = new MyCodec[Person] {}
+16 |  override def toString = ""

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/neg/i22589c.check

@@ -0,0 +1,7 @@
+-- Error: tests/neg/i22589c.scala:8:7 ----------------------------------------------------------------------------------
+ 8 |object A extends Base[A.P] { // error
+   |^
+   |Inferred implementation of the deferred given instance given_TC_T in trait Base is self-recursive.
+   |An implementing given needs to be written explicitly.
+ 9 |  case class P()
+10 |}

tests/neg/i22589c.scala

@@ -0,0 +1,10 @@
+
+trait TC[T]
+
+trait Base[T] {
+  given TC[T] = scala.compiletime.deferred
+}
+
+object A extends Base[A.P] { // error
+  case class P()
+}

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 = ""