warn when anonymous mirror.sum generated

Author: bishabosha

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -169,7 +169,8 @@ enum ErrorMessageID extends java.lang.Enum[ErrorMessageID] {
     InvalidReferenceInImplicitNotFoundAnnotationID,
     TraitMayNotDefineNativeMethodID,
     JavaEnumParentArgsID,
-    AlreadyDefinedID
+    AlreadyDefinedID,
+    MissingCompanionForMirrorID
 
   def errorNumber = ordinal - 2
 }

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -235,6 +235,23 @@ import transform.SymUtils._
     }
   }
 
+
+
+  class MissingCompanionForMirror(cls: Symbol, isSum: Boolean)(using Context)
+  extends DeclarationMsg(MissingCompanionForMirrorID) {
+    def msg = em"""|an anonymous ${if isSum then "sum" else "product"} type Mirror will be generated for $cls,
+                   |which does not declare a companion object.""".stripMargin
+    def explain =
+      em"""|$cls is a type declared without a companion object.
+           |This means that summoning its Mirror from the context
+           |will synthesize a new anonymous instance at each callsite.
+           |In general, a class under the control of the Scala compiler
+           |will have its Mirror cached in its companion object,
+           |if one exists, leading to fewer allocations.
+           |Adding a deriving clause to $cls will create a
+           |companion object even if one is not explicitly declared.""".stripMargin
+  }
+
   class TypeMismatch(found: Type, expected: Type, addenda: => String*)(using Context)
     extends TypeMismatchMsg(TypeMismatchID):
 

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

@@ -321,7 +321,10 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
           .refinedWith(tpnme.MirroredElemLabels, TypeAlias(TypeOps.nestedPairs(elemLabels)))
       val mirrorRef =
         if useCompanion then companionPath(mirroredType, span)
-        else anonymousMirror(monoType, ExtendsSumMirror, span)
+        else
+          if !cls.is(Scala2x) then
+            report.warning(new reporting.MissingCompanionForMirror(cls, isSum = true), ctx.source.atSpan(span))
+          anonymousMirror(monoType, ExtendsSumMirror, span)
       mirrorRef.cast(mirrorType)
     else EmptyTree
   end sumMirror

tests/neg-custom-args/fatal-warnings/i10997.scala

@@ -0,0 +1,12 @@
+class Test {
+
+  sealed trait Parent
+  case class Foo(x: Int, y: Int, s: String) extends Parent
+  case class Bar(x: Int, y: Int) extends Parent
+
+  println(summon[deriving.Mirror.Of[Parent]]) // error: anonymous Mirror generated.
+}
+
+class Test2 {
+  println(summon[deriving.Mirror.Of[List[Int]]]) // ok: List is Scala 2 defined
+}