Try to be more subtle when inferring type parameters of class parents

The previous scheme was basically like this:

 1. If the parent type is an application, infer type parameters from the
   term parameters during completion of the class.
 2. If the parent type is an Ident or Select and it appears in an anonymous
   class, infer it from the expected type.

It could happen in the second case that we infer Nothing since an expected type
was missing, but if the parent type had been an application, we would have inferred
something else from implicit arguments to the parent constructor. This is a case
handled by Scala 2, but not yet by Dotty.

To deal with this we have to perform a complicated dance:

 - Try step (2) above, but back out if the inferred parent type has Nothing
   as a type argument.
 - During completion, if the inferred parent has missing type arguments, convert
   the parent type to an application with () arguments and try that instead.

I normally would have thought this is too much sophistry but there are valid
use cases that Scala 2 supports and it would be good if we get to parity for these.

Author: odersky

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

@@ -307,16 +307,17 @@ object Inferencing {
   }
 
   /** If `tree` has a type lambda type, infer its type parameters by comparing with expected type `pt` */
-  def inferTypeParams(tree: Tree, pt: Type)(using Context): Tree = tree.tpe match {
+  def inferTypeParams(tree: Tree, pt: Type)(using Context): Tree = tree.tpe match
     case tl: TypeLambda =>
       val (tl1, tvars) = constrained(tl, tree)
       var tree1 = AppliedTypeTree(tree.withType(tl1), tvars)
       tree1.tpe <:< pt
-      fullyDefinedType(tree1.tpe, "template parent", tree.srcPos)
-      tree1
+      if isFullyDefined(tree1.tpe, force = ForceDegree.failBottom) then
+        tree1
+      else
+        EmptyTree
     case _ =>
       tree
-  }
 
   def isSkolemFree(tp: Type)(using Context): Boolean =
     !tp.existsPart(_.isInstanceOf[SkolemType])

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

@@ -1453,27 +1453,41 @@ class Namer { typer: Typer =>
        * only if parent type contains uninstantiated type parameters.
        */
       def parentType(parent: untpd.Tree)(using Context): Type =
-        if (parent.isType)
-          typedAheadType(parent, AnyTypeConstructorProto).tpe
-        else {
-          val (core, targs) = stripApply(parent) match {
+
+        def typedParentApplication(parent: untpd.Tree): Type =
+          val (core, targs) = stripApply(parent) match
             case TypeApply(core, targs) => (core, targs)
             case core => (core, Nil)
-          }
-          core match {
+          core match
             case Select(New(tpt), nme.CONSTRUCTOR) =>
               val targs1 = targs map (typedAheadType(_))
               val ptype = typedAheadType(tpt).tpe appliedTo targs1.tpes
               if (ptype.typeParams.isEmpty) ptype
-              else {
+              else
                 if (denot.is(ModuleClass) && denot.sourceModule.isOneOf(GivenOrImplicit))
                   missingType(denot.symbol, "parent ")(using creationContext)
                 fullyDefinedType(typedAheadExpr(parent).tpe, "class parent", parent.srcPos)
-              }
             case _ =>
               UnspecifiedErrorType.assertingErrorsReported
-          }
-        }
+
+        def typedParentType(tree: untpd.Tree): tpd.Tree =
+          val parentTpt = typer.typedType(parent, AnyTypeConstructorProto)
+          val ptpe = parentTpt.tpe
+          if ptpe.typeParams.nonEmpty
+              && ptpe.underlyingClassRef(refinementOK = false).exists
+          then
+            // Try to infer type parameters from a synthetic application.
+            // This might yield new info if implicit parameters are resolved.
+            // A test case is i16778.scala.
+            val app = untpd.Apply(untpd.Select(untpd.New(parentTpt), nme.CONSTRUCTOR), Nil)
+            typedParentApplication(app)
+            app.getAttachment(TypedAhead).getOrElse(parentTpt)
+          else
+            parentTpt
+
+        if parent.isType then typedAhead(parent, typedParentType).tpe
+        else typedParentApplication(parent)
+      end parentType
 
       /** Check parent type tree `parent` for the following well-formedness conditions:
        *  (1) It must be a class type with a stable prefix (@see checkClassTypeWithStablePrefix)
@@ -1607,7 +1621,7 @@ class Namer { typer: Typer =>
       case Some(ttree) => ttree
       case none =>
         val ttree = typed(tree)
-        xtree.putAttachment(TypedAhead, ttree)
+        if !ttree.isEmpty then xtree.putAttachment(TypedAhead, ttree)
         ttree
     }
   }

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

@@ -843,14 +843,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             isSkolemFree(pt) &&
             isEligible(pt.underlyingClassRef(refinementOK = false)))
           templ1 = cpy.Template(templ)(parents = untpd.TypeTree(pt) :: Nil)
-        templ1.parents foreach {
-          case parent: RefTree =>
-            typedAhead(parent, tree => inferTypeParams(typedType(tree), pt))
-          case _ =>
-        }
-        val x = tpnme.ANON_CLASS
-        val clsDef = TypeDef(x, templ1).withFlags(Final | Synthetic)
-        typed(cpy.Block(tree)(clsDef :: Nil, New(Ident(x), Nil)), pt)
+        for case parent: RefTree <- templ1.parents do
+          typedAhead(parent, tree => inferTypeParams(typedType(tree), pt))
+        val anon = tpnme.ANON_CLASS
+        val clsDef = TypeDef(anon, templ1).withFlags(Final | Synthetic)
+        typed(cpy.Block(tree)(clsDef :: Nil, New(Ident(anon), Nil)), pt)
       case _ =>
         var tpt1 = typedType(tree.tpt)
         val tsym = tpt1.tpe.underlyingClassRef(refinementOK = false).typeSymbol

tests/neg/i1643.scala

@@ -1,4 +1,4 @@
-trait T extends Array { // error // error
+trait T extends Array { // error
   def t1(as: String*): Array[String] = { varargs1(as*) } // error
   def t2(as: String*): Array[String] = { super.varargs1(as*) } // error
 }
@@ -7,7 +7,7 @@ class C extends Base_1 { // error
   def c2(as: String*): Array[String] = { super.varargs1(as*) } // error
 }
 object Test extends App {
-  val t = new T {}  // error
+  val t = new T {}
   println(t.t1("a", "b").mkString(","))
   println(t.t2("a", "b").mkString(","))
   val c = new C {}

tests/neg/i4820.scala

@@ -0,0 +1,22 @@
+final abstract class ForcedRecompilationToken[T]
+
+object ForcedRecompilationToken {
+  implicit def materialize: ForcedRecompilationToken["x"] = null.asInstanceOf[ForcedRecompilationToken["x"]]
+}
+
+class PluginDef[T](implicit val recompilationToken: ForcedRecompilationToken[T])
+
+object X {
+  val no = {
+    final class anon extends PluginDef {} // was: missing type parameters
+    new anon
+  }
+
+  val bad = new PluginDef {} // was: No given instance
+  val good = new PluginDef() {} // ok
+}
+
+object DependingPlugin {
+  class NestedDoublePlugin extends PluginDef
+  object NestedDoublePlugin extends PluginDef
+}

tests/neg/i4820b.scala

@@ -0,0 +1,2 @@
+class Foo[A]
+class Bar[A] extends Foo  // was error, now expanded to Foo[Nothing]

tests/neg/i4820c.scala

@@ -0,0 +1,5 @@
+trait SetOps[A, +C <: SetOps[A, C]]  {
+  def concat(that: Iterable[A]): C = ???
+}
+
+class Set1[A] extends SetOps // ideally should be SetOps[A, Set1[A]], but SetOps[Nothing, Nothin] is inferred