Warn recursive deferred given impl selects itself

Author: som-snytt

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

@@ -3115,52 +3115,55 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
      *  parameters of the current class are also defined.
      */
     def implementDeferredGivens(body: List[Tree]): List[Tree] =
+      def failFor(mbr: TermRef, why: String): false =
+        report.error(
+          em"""Cannot infer the implementation of the deferred ${mbr.symbol.showLocated}
+              |since $why. An implementing given needs to be written explicitly.""",
+          cdef.srcPos)
+        false
+      def isGivenValue(mbr: TermRef) = !mbr.symbol.is(Method) || failFor(mbr, "that given is parameterized")
+
+      def willBeImplementedInParentClass(m: TermRef) =
+        val superCls = cls.superClass
+        superCls.exists && superCls.asClass.baseClasses.contains(m.symbol.owner)
+
+      def givenImpl(mbr: TermRef): ValDef =
+        val dcl = mbr.symbol
+        val target = dcl.info.asSeenFrom(cls.thisType, dcl.owner)
+        val constr = cls.primaryConstructor
+        val usingParamAccessors = cls.paramAccessors.filter(_.is(Given))
+        val paramScope = newScopeWith(usingParamAccessors*)
+        val searchCtx = ctx.outer.fresh.setScope(paramScope)
+        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,
+          info = target,
+          coord = rhs.span).entered.asTerm
+
+        def anchorParams = new TreeMap:
+          override def transform(tree: Tree)(using Context): Tree = tree match
+            case id: Ident if usingParamAccessors.contains(id.symbol) =>
+              cpy.Select(id)(This(cls), id.name)
+            case _ =>
+              super.transform(tree)
+        ValDef(impl, anchorParams.transform(rhs)).withSpan(impl.span.endPos)
+      end givenImpl
+
       if cls.is(Trait) || ctx.isAfterTyper then body
       else
-        def isGivenValue(mbr: TermRef) =
-          val dcl = mbr.symbol
-          if dcl.is(Method) then
-            report.error(
-              em"""Cannnot infer the implementation of the deferred ${dcl.showLocated}
-                  |since that given is parameterized. An implementing given needs to be written explicitly.""",
-              cdef.srcPos)
-            false
-          else true
-
-        def willBeimplementedInParentClass(m: TermRef) =
-          val superCls = cls.superClass
-          superCls.exists && superCls.asClass.baseClasses.contains(m.symbol.owner)
-
-        def givenImpl(mbr: TermRef): ValDef =
-          val dcl = mbr.symbol
-          val target = dcl.info.asSeenFrom(cls.thisType, dcl.owner)
-          val constr = cls.primaryConstructor
-          val usingParamAccessors = cls.paramAccessors.filter(_.is(Given))
-          val paramScope = newScopeWith(usingParamAccessors*)
-          val searchCtx = ctx.outer.fresh.setScope(paramScope)
-          val rhs = implicitArgTree(target, cdef.span,
-              where = i"inferring the implementation of the deferred ${dcl.showLocated}"
-            )(using searchCtx)
-
-          val impl = dcl.copy(cls,
-            flags = dcl.flags &~ (HasDefault | Deferred) | Final | Override,
-            info = target,
-            coord = rhs.span).entered.asTerm
-
-          def anchorParams = new TreeMap:
-            override def transform(tree: Tree)(using Context): Tree = tree match
-              case id: Ident if usingParamAccessors.contains(id.symbol) =>
-                cpy.Select(id)(This(cls), id.name)
-              case _ =>
-                super.transform(tree)
-          ValDef(impl, anchorParams.transform(rhs)).withSpan(impl.span.endPos)
-        end givenImpl
-
         val givenImpls =
           cls.thisType.implicitMembers
             //.showing(i"impl def givens for $cls/$result")
             .filter(_.symbol.isAllOf(DeferredGivenFlags, butNot = Param))
-            .filter(!willBeimplementedInParentClass(_)) // only implement the given in the topmost class
+            .filter(!willBeImplementedInParentClass(_)) // only implement the given in the topmost class
             //.showing(i"impl def filtered givens for $cls/$result")
             .filter(isGivenValue)
             .map(givenImpl)

tests/neg/deferred-givens.check

@@ -9,5 +9,5 @@
 -- Error: tests/neg/deferred-givens.scala:26:8 -------------------------------------------------------------------------
 26 |  class E extends A2  // error, can't summon polymorphic given
    |  ^^^^^^^^^^^^^^^^^^
-   |  Cannnot infer the implementation of the deferred given instance given_Ctx3_T in trait A2
+   |  Cannot infer the implementation of the deferred given instance given_Ctx3_T in trait A2
    |  since that given is parameterized. An implementing given needs to be written explicitly.

tests/neg/i22589.check

@@ -0,0 +1,7 @@
+-- Error: tests/neg/i22589.scala:15:7 ----------------------------------------------------------------------------------
+15 |object Person extends CompanionEssentials[Person]: // error
+   |^
+   |Cannot infer the implementation of the deferred given instance given_MyCodec_E in trait CompanionEssentials
+   |since the result 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
+   |^
+   |Cannot infer the implementation of the deferred given instance myc in trait CompanionEssentials
+   |since the result 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/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 = ""