Avoid follow-on error when trying to resolve overloads

resolve overload errors should be suppressed if some types are
already erroenous.

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -280,7 +280,7 @@ object Types {
     final def isError(implicit ctx: Context): Boolean = stripTypeVar.isInstanceOf[ErrorType]
 
     /** Is some part of this type produced as a repair for an error? */
-    final def isErroneous(implicit ctx: Context): Boolean = existsPart(_.isError, forceLazy = false)
+    def isErroneous(implicit ctx: Context): Boolean = existsPart(_.isError, forceLazy = false)
 
     /** Does the type carry an annotation that is an instance of `cls`? */
     @tailrec final def hasAnnotation(cls: ClassSymbol)(implicit ctx: Context): Boolean = stripTypeVar match {

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

@@ -333,6 +333,9 @@ object ProtoTypes {
 
     def isDropped: Boolean = state.toDrop
 
+    override def isErroneous(implicit ctx: Context): Boolean =
+      state.typedArgs.tpes.exists(_.widen.isErroneous)
+
     override def toString: String = s"FunProto(${args mkString ","} => $resultType)"
 
     def map(tm: TypeMap)(implicit ctx: Context): FunProto =

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

@@ -232,21 +232,21 @@ trait TypeAssigner {
    */
   def selectionType(site: Type, name: Name, pos: Position)(implicit ctx: Context): Type = {
     val mbr = site.member(name)
-    if (reallyExists(mbr)) site.select(name, mbr)
-    else if (site.derivesFrom(defn.DynamicClass) && !Dynamic.isDynamicMethod(name)) {
+    if (reallyExists(mbr))
+      site.select(name, mbr)
+    else if (site.derivesFrom(defn.DynamicClass) && !Dynamic.isDynamicMethod(name))
       TryDynamicCallType
-    } else {
-      if (site.isErroneous || name.toTermName == nme.ERROR) UnspecifiedErrorType
-      else {
-        def kind = if (name.isTypeName) "type" else "value"
-        def addendum =
-          if (site.derivesFrom(defn.DynamicClass)) "\npossible cause: maybe a wrong Dynamic method signature?"
-          else ""
-        errorType(
-          if (name == nme.CONSTRUCTOR) ex"$site does not have a constructor"
-          else NotAMember(site, name, kind),
-          pos)
-      }
+    else if (site.isErroneous || name.toTermName == nme.ERROR)
+      UnspecifiedErrorType
+    else {
+      def kind = if (name.isTypeName) "type" else "value"
+      def addendum =
+        if (site.derivesFrom(defn.DynamicClass)) "\npossible cause: maybe a wrong Dynamic method signature?"
+        else ""
+      errorType(
+        if (name == nme.CONSTRUCTOR) ex"$site does not have a constructor"
+        else NotAMember(site, name, kind),
+        pos)
     }
   }
 

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

@@ -2236,8 +2236,11 @@ class Typer extends Namer
                   noMatches
             }
           case alts =>
-            val remainingDenots = alts map (_.denot.asInstanceOf[SingleDenotation])
-            errorTree(tree, AmbiguousOverload(tree, remainingDenots, pt)(err))
+            if (tree.tpe.isErroneous || pt.isErroneous) tree.withType(UnspecifiedErrorType)
+            else {
+              val remainingDenots = alts map (_.denot.asInstanceOf[SingleDenotation])
+              errorTree(tree, AmbiguousOverload(tree, remainingDenots, pt)(err))
+            }
         }
       }