Generate lazy vals for parameterless classes in deriving

Author: odersky

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

@@ -71,13 +71,16 @@ class CacheAliasImplicits extends MiniPhase with IdentityDenotTransformer { this
         case _ => false
       }
     }
-    if (isCached) {
-      sym.copySymDenotation(
-        initFlags = sym.flags &~ Method | Lazy,
-        info = sym.info.widenExpr)
-      .installAfter(thisPhase)
-      cpy.ValDef(tree)(tree.name, tree.tpt, tree.rhs)
-    }
+    if isCached then
+      if true then
+        println(i"NOT CACHING $tree")
+        tree
+      else
+        sym.copySymDenotation(
+          initFlags = sym.flags &~ Method | Lazy,
+          info = sym.info.widenExpr)
+        .installAfter(thisPhase)
+        cpy.ValDef(tree)(tree.name, tree.tpt, tree.rhs)
     else tree
   }
 

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

@@ -54,8 +54,9 @@ trait Deriving {
         // If we set the Synthetic flag here widenGiven will widen too far and the
         // derived instance will have too low a priority to be selected over a freshly
         // derived instance at the summoning site.
+        val flags = if info.isInstanceOf[MethodOrPoly] then Given | Method else Given | Lazy
         synthetics +=
-          newSymbol(ctx.owner, instanceName, Given | Method, info, coord = pos.span)
+          newSymbol(ctx.owner, instanceName, flags, info, coord = pos.span)
             .entered
     }
 
@@ -96,8 +97,7 @@ trait Deriving {
       def addInstance(derivedParams: List[TypeSymbol], evidenceParamInfos: List[List[Type]], instanceTypes: List[Type]): Unit = {
         val resultType = typeClassType.appliedTo(instanceTypes)
         val monoInfo =
-          if evidenceParamInfos.isEmpty then
-            if derivedParams.isEmpty then ExprType(resultType) else resultType
+          if evidenceParamInfos.isEmpty then resultType
           else ImplicitMethodType(evidenceParamInfos.map(typeClassType.appliedTo), resultType)
         val derivedInfo =
           if derivedParams.isEmpty then monoInfo
@@ -295,7 +295,8 @@ trait Deriving {
       }
 
       def syntheticDef(sym: Symbol): Tree = inContext(ctx.fresh.setOwner(sym).setNewScope) {
-        tpd.polyDefDef(sym.asTerm, typeclassInstance(sym))
+        if sym.is(Method) then tpd.polyDefDef(sym.asTerm, typeclassInstance(sym))
+        else tpd.ValDef(sym.asTerm, typeclassInstance(sym)(Nil)(Nil))
       }
 
       synthetics.map(syntheticDef).toList