Make errors are not swept under the carpet

Typer#ensureReported's comment outlines an example where errors
could go unreported, resulting in error trees after typer without
any reported error messages. This commit makes sure that at least
one error is reported if a tree node has an error type.

Fixes #1802.

18 files changed, 78 insertions, 55 deletions

diff --git a/compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala b/compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala
index ed32d2df9..123dc3450 100644
--- a/compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala
+++ b/compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala
@@ -1024,7 +1024,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
       case JavaArrayType(elem) => elem
       case _ =>
         ctx.error(s"JavaSeqArray with type ${field.tpe} reached backend: $field", field.pos)
-        ErrorType
+        UnspecifiedErrorType
     }
     def _2: List[Tree] = field.elems
   }
diff --git a/compiler/src/dotty/tools/backend/jvm/scalaPrimitives.scala b/compiler/src/dotty/tools/backend/jvm/scalaPrimitives.scala
index eb4da5400..89831e56b 100644
--- a/compiler/src/dotty/tools/backend/jvm/scalaPrimitives.scala
+++ b/compiler/src/dotty/tools/backend/jvm/scalaPrimitives.scala
@@ -12,7 +12,7 @@ import dotty.tools.dotc.ast.tpd._
 import dotty.tools.dotc.core.Names.TermName
 import dotty.tools.dotc.core.StdNames
 import dotty.tools.dotc.core.StdNames._
-import dotty.tools.dotc.core.Types.{JavaArrayType, ErrorType, Type}
+import dotty.tools.dotc.core.Types.{JavaArrayType, UnspecifiedErrorType, Type}
 
 import scala.collection.{ mutable, immutable }
 
@@ -73,7 +73,7 @@ class DottyPrimitives(ctx: Context) {
       case JavaArrayType(el) => el
       case _ =>
         ctx.error(s"expected Array $tpe")
-        ErrorType
+        UnspecifiedErrorType
     }
 
     code match {
diff --git a/compiler/src/dotty/tools/dotc/ast/Trees.scala b/compiler/src/dotty/tools/dotc/ast/Trees.scala
index 20273eb85..28ae89c4a 100644
--- a/compiler/src/dotty/tools/dotc/ast/Trees.scala
+++ b/compiler/src/dotty/tools/dotc/ast/Trees.scala
@@ -113,7 +113,7 @@ object Trees {
      *  type. (Overridden by empty trees)
      */
     def withType(tpe: Type)(implicit ctx: Context): ThisTree[Type] = {
-      if (tpe == ErrorType) assert(ctx.reporter.errorsReported)
+      if (tpe.isInstanceOf[ErrorType]) assert(ctx.reporter.errorsReported)
       withTypeUnchecked(tpe)
     }
 
diff --git a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
index e2bb0ac1a..aaae78c57 100644
--- a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -1937,12 +1937,12 @@ object SymDenotations {
   /** A completer for missing references */
   class StubInfo() extends LazyType {
 
-    def initializeToDefaults(denot: SymDenotation)(implicit ctx: Context) = {
+    def initializeToDefaults(denot: SymDenotation, errMsg: => String)(implicit ctx: Context) = {
       denot.info = denot match {
         case denot: ClassDenotation =>
           ClassInfo(denot.owner.thisType, denot.classSymbol, Nil, EmptyScope)
         case _ =>
-          ErrorType
+          new ErrorType(errMsg)
       }
       denot.privateWithin = NoSymbol
     }
@@ -1954,13 +1954,14 @@ object SymDenotations {
         if (file != null) (s" in $file", file.toString)
         else ("", "the signature")
       val name = ctx.fresh.setSetting(ctx.settings.debugNames, true).nameString(denot.name)
-      ctx.error(
+      def errMsg =
         i"""bad symbolic reference. A signature$location
            |refers to $name in ${denot.owner.showKind} ${denot.owner.showFullName} which is not available.
            |It may be completely missing from the current classpath, or the version on
-           |the classpath might be incompatible with the version used when compiling $src.""")
+           |the classpath might be incompatible with the version used when compiling $src."""
+      ctx.error(errMsg)
       if (ctx.debug) throw new Error()
-      initializeToDefaults(denot)
+      initializeToDefaults(denot, errMsg)
     }
   }
 
diff --git a/compiler/src/dotty/tools/dotc/core/Symbols.scala b/compiler/src/dotty/tools/dotc/core/Symbols.scala
index cfd85c49c..b10b94ad8 100644
--- a/compiler/src/dotty/tools/dotc/core/Symbols.scala
+++ b/compiler/src/dotty/tools/dotc/core/Symbols.scala
@@ -22,6 +22,7 @@ import NameOps._
 import ast.tpd.Tree
 import ast.TreeTypeMap
 import Constants.Constant
+import reporting.diagnostic.Message
 import Denotations.{ Denotation, SingleDenotation, MultiDenotation }
 import collection.mutable
 import io.AbstractFile
@@ -290,9 +291,11 @@ trait Symbols { this: Context =>
    */
   def newSkolem(tp: Type) = newSymbol(defn.RootClass, nme.SKOLEM, SyntheticArtifact | Permanent, tp)
 
-  def newErrorSymbol(owner: Symbol, name: Name) =
+  def newErrorSymbol(owner: Symbol, name: Name, msg: => Message) = {
+    val errType = new ErrorType(msg)
     newSymbol(owner, name, SyntheticArtifact,
-      if (name.isTypeName) TypeAlias(ErrorType) else ErrorType)
+        if (name.isTypeName) TypeAlias(errType) else errType)
+  }
 
   /** Map given symbols, subjecting their attributes to the mappings
    *  defined in the given TreeTypeMap `ttmap`.
diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index 743220f55..e8905320b 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -265,7 +265,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
         case ConstantType(v1) => v1.value == v2.value
         case _ => secondTry(tp1, tp2)
       }
-    case ErrorType =>
+    case _: FlexType =>
       true
     case _ =>
       secondTry(tp1, tp2)
@@ -341,7 +341,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
           false
       }
       joinOK || isSubType(tp11, tp2) && isSubType(tp12, tp2)
-    case ErrorType =>
+    case _: FlexType =>
       true
     case _ =>
       thirdTry(tp1, tp2)
diff --git a/compiler/src/dotty/tools/dotc/core/TypeErasure.scala b/compiler/src/dotty/tools/dotc/core/TypeErasure.scala
index 82943377a..2e7c1db04 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeErasure.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeErasure.scala
@@ -55,7 +55,7 @@ object TypeErasure {
       tp.paramTypes.forall(isErasedType) && isErasedType(tp.resultType)
     case tp @ ClassInfo(pre, _, parents, decls, _) =>
       isErasedType(pre) && parents.forall(isErasedType) //&& decls.forall(sym => isErasedType(sym.info)) && isErasedType(tp.selfType)
-    case NoType | NoPrefix | WildcardType | ErrorType | SuperType(_, _) =>
+    case NoType | NoPrefix | WildcardType | _: ErrorType | SuperType(_, _) =>
       true
     case _ =>
       false
@@ -398,7 +398,7 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
         tp.derivedClassInfo(NoPrefix, parents, erasedDecls, erasedRef(tp.selfType))
           // can't replace selftype by NoType because this would lose the sourceModule link
       }
-    case NoType | NoPrefix | ErrorType | JavaArrayType(_) =>
+    case NoType | NoPrefix | _: ErrorType | JavaArrayType(_) =>
       tp
     case tp: WildcardType if wildcardOK =>
       tp
@@ -506,7 +506,7 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
         if (inst.exists) sigName(inst) else tpnme.Uninstantiated
       case tp: TypeProxy =>
         sigName(tp.underlying)
-      case ErrorType | WildcardType =>
+      case _: ErrorType | WildcardType =>
         tpnme.WILDCARD
       case tp: WildcardType =>
         sigName(tp.optBounds)
diff --git a/compiler/src/dotty/tools/dotc/core/TypeOps.scala b/compiler/src/dotty/tools/dotc/core/TypeOps.scala
index 39214dd0c..f134412a7 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeOps.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeOps.scala
@@ -433,7 +433,7 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
               formals = formals.updated(name, tp1.typeParamNamed(name))
           }
           normalizeToRef(tp1)
-        case ErrorType =>
+        case _: ErrorType =>
           defn.AnyType
         case AnnotatedType(tpe, _) =>
           normalizeToRef(tpe)
diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index 4dffc4542..8f43ed9bb 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -15,9 +15,10 @@ import SymDenotations._
 import Decorators._
 import Denotations._
 import Periods._
-import util.Positions.Position
+import util.Positions.{Position, NoPosition}
 import util.Stats._
 import util.{DotClass, SimpleMap}
+import reporting.diagnostic.Message
 import ast.tpd._
 import ast.TreeTypeMap
 import printing.Texts._
@@ -176,7 +177,7 @@ object Types {
 
     /** Is this type produced as a repair for an error? */
     final def isError(implicit ctx: Context): Boolean = stripTypeVar match {
-      case ErrorType => true
+      case _: ErrorType => true
       case tp => (tp.typeSymbol is Erroneous) || (tp.termSymbol is Erroneous)
     }
 
@@ -387,8 +388,8 @@ object Types {
         tp.decls.denotsNamed(name).filterExcluded(excluded).toDenot(NoPrefix)
       case tp: TypeProxy =>
         tp.underlying.findDecl(name, excluded)
-      case ErrorType =>
-        ctx.newErrorSymbol(classSymbol orElse defn.RootClass, name)
+      case err: ErrorType =>
+        ctx.newErrorSymbol(classSymbol orElse defn.RootClass, name, err.msg)
       case _ =>
         NoDenotation
     }
@@ -453,8 +454,8 @@ object Types {
           go(tp.join)
         case tp: JavaArrayType =>
           defn.ObjectType.findMember(name, pre, excluded)
-        case ErrorType =>
-          ctx.newErrorSymbol(pre.classSymbol orElse defn.RootClass, name)
+        case err: ErrorType =>
+          ctx.newErrorSymbol(pre.classSymbol orElse defn.RootClass, name, err.msg)
         case _ =>
           NoDenotation
       }
@@ -1497,7 +1498,7 @@ object Types {
           (lastDefRunId != sym.defRunId) ||
           (lastDefRunId == NoRunId)
         } ||
-        (lastSymbol.infoOrCompleter == ErrorType ||
+        (lastSymbol.infoOrCompleter.isInstanceOf[ErrorType] ||
         sym.owner != lastSymbol.owner &&
         (sym.owner.derivesFrom(lastSymbol.owner) ||
          selfTypeOf(sym).derivesFrom(lastSymbol.owner) ||
@@ -2693,7 +2694,7 @@ object Types {
     protected def checkInst(implicit ctx: Context): this.type = {
       def check(tycon: Type): Unit = tycon.stripTypeVar match {
         case tycon: TypeRef if !tycon.symbol.isClass =>
-        case _: PolyParam | ErrorType | _: WildcardType =>
+        case _: PolyParam | _: ErrorType | _: WildcardType =>
         case _: PolyType =>
           assert(args.exists(_.isInstanceOf[TypeBounds]), s"unreduced type apply: $this")
         case tycon: AnnotatedType =>
@@ -3251,12 +3252,16 @@ object Types {
     override def computeHash = hashSeed
   }
 
-  abstract class ErrorType extends UncachedGroundType with ValueType
+  abstract class FlexType extends UncachedGroundType with ValueType
 
-  object ErrorType extends ErrorType
+  class ErrorType(_msg: => Message) extends FlexType {
+    val msg = _msg
+  }
+
+  object UnspecifiedErrorType extends ErrorType("unspecified error")
 
   /* Type used to track Select nodes that could not resolve a member and their qualifier is a scala.Dynamic. */
-  object TryDynamicCallType extends ErrorType
+  object TryDynamicCallType extends FlexType
 
   /** Wildcard type, possibly with bounds */
   abstract case class WildcardType(optBounds: Type) extends CachedGroundType with TermType {
diff --git a/compiler/src/dotty/tools/dotc/parsing/JavaParsers.scala b/compiler/src/dotty/tools/dotc/parsing/JavaParsers.scala
index 0f63b25bb..92ab10db9 100644
--- a/compiler/src/dotty/tools/dotc/parsing/JavaParsers.scala
+++ b/compiler/src/dotty/tools/dotc/parsing/JavaParsers.scala
@@ -90,7 +90,8 @@ object JavaParsers {
       if (skipIt)
         skip()
     }
-    def errorTypeTree = TypeTree().withType(ErrorType) withPos Position(in.offset)
+
+    def errorTypeTree = TypeTree().withType(UnspecifiedErrorType) withPos Position(in.offset)
 
     // --------- tree building -----------------------------
 
diff --git a/compiler/src/dotty/tools/dotc/printing/Formatting.scala b/compiler/src/dotty/tools/dotc/printing/Formatting.scala
index b321d3736..05f1af9d7 100644
--- a/compiler/src/dotty/tools/dotc/printing/Formatting.scala
+++ b/compiler/src/dotty/tools/dotc/printing/Formatting.scala
@@ -96,7 +96,10 @@ object Formatting {
       case tpe: Type =>
         tpe.exists && !tpe.isErroneous
       case sym: Symbol if sym.isCompleted =>
-        sym.info != ErrorType && sym.info != TypeAlias(ErrorType) && sym.info.exists
+        sym.info match {
+          case _: ErrorType | TypeAlias(_: ErrorType) | NoType => false
+          case _ => true
+        }
       case _ => true
     }
 
diff --git a/compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala b/compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala
index 15c382bb0..61f23c214 100644
--- a/compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala
+++ b/compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala
@@ -154,7 +154,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
         changePrec(AndPrec) { toText(tp1) ~ " & " ~ toText(tp2) }
       case OrType(tp1, tp2) =>
         changePrec(OrPrec) { toText(tp1) ~ " | " ~ toText(tp2) }
-      case ErrorType =>
+      case _: ErrorType =>
         "<error>"
       case tp: WildcardType =>
         if (tp.optBounds.exists) "(?" ~ toTextRHS(tp.bounds) ~ ")" else "?"
diff --git a/compiler/src/dotty/tools/dotc/typer/Applications.scala b/compiler/src/dotty/tools/dotc/typer/Applications.scala
index 4203ab9b2..da0a59c7b 100644
--- a/compiler/src/dotty/tools/dotc/typer/Applications.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Applications.scala
@@ -561,7 +561,7 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
       var typedArgs = typedArgBuf.toList
       def app0 = cpy.Apply(app)(normalizedFun, typedArgs) // needs to be a `def` because typedArgs can change later
       val app1 =
-        if (!success) app0.withType(ErrorType)
+        if (!success) app0.withType(UnspecifiedErrorType)
         else {
           if (!sameSeq(args, orderedArgs)) {
             // need to lift arguments to maintain evaluation order in the
@@ -654,7 +654,7 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
         }
 
       fun1.tpe match {
-        case ErrorType => untpd.cpy.Apply(tree)(fun1, tree.args).withType(ErrorType)
+        case err: ErrorType => untpd.cpy.Apply(tree)(fun1, tree.args).withType(err)
         case TryDynamicCallType => typedDynamicApply(tree, pt)
         case _ =>
           tryEither {
@@ -918,7 +918,7 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
       case tp =>
         val unapplyErr = if (tp.isError) unapplyFn else notAnExtractor(unapplyFn)
         val typedArgsErr = args mapconserve (typed(_, defn.AnyType))
-        cpy.UnApply(tree)(unapplyErr, Nil, typedArgsErr) withType ErrorType
+        cpy.UnApply(tree)(unapplyErr, Nil, typedArgsErr) withType unapplyErr.tpe
     }
   }
 
diff --git a/compiler/src/dotty/tools/dotc/typer/Checking.scala b/compiler/src/dotty/tools/dotc/typer/Checking.scala
index f5f7bdbaa..41d9f9572 100644
--- a/compiler/src/dotty/tools/dotc/typer/Checking.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Checking.scala
@@ -56,7 +56,7 @@ object Checking {
   def checkBounds(args: List[tpd.Tree], poly: PolyType)(implicit ctx: Context): Unit =
     checkBounds(args, poly.paramBounds, _.substParams(poly, _))
 
-  /** Check applied type trees for well-formedness. This means 
+  /** Check applied type trees for well-formedness. This means
    *   - all arguments are within their corresponding bounds
    *   - if type is a higher-kinded application with wildcard arguments,
    *     check that it or one of its supertypes can be reduced to a normal application.
@@ -237,8 +237,7 @@ object Checking {
     catch {
       case ex: CyclicReference =>
         if (reportErrors) {
-          ctx.error(i"illegal cyclic reference: ${checker.where} ${checker.lastChecked} of $sym refers back to the type itself", sym.pos)
-          ErrorType
+          errorType(i"illegal cyclic reference: ${checker.where} ${checker.lastChecked} of $sym refers back to the type itself", sym.pos)
         }
         else info
     }
diff --git a/compiler/src/dotty/tools/dotc/typer/Dynamic.scala b/compiler/src/dotty/tools/dotc/typer/Dynamic.scala
index b5ace87d3..4039c8b81 100644
--- a/compiler/src/dotty/tools/dotc/typer/Dynamic.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Dynamic.scala
@@ -11,6 +11,7 @@ import dotty.tools.dotc.core.Names.Name
 import dotty.tools.dotc.core.StdNames._
 import dotty.tools.dotc.core.Types._
 import dotty.tools.dotc.core.Decorators._
+import ErrorReporting._
 
 object Dynamic {
   def isDynamicMethod(name: Name): Boolean =
@@ -41,10 +42,9 @@ trait Dynamic { self: Typer with Applications =>
       def isNamedArg(arg: untpd.Tree): Boolean = arg match { case NamedArg(_, _) => true; case _ => false }
       val args = tree.args
       val dynName = if (args.exists(isNamedArg)) nme.applyDynamicNamed else nme.applyDynamic
-      if (dynName == nme.applyDynamicNamed && untpd.isWildcardStarArgList(args)) {
-        ctx.error("applyDynamicNamed does not support passing a vararg parameter", tree.pos)
-        tree.withType(ErrorType)
-      } else {
+      if (dynName == nme.applyDynamicNamed && untpd.isWildcardStarArgList(args))
+        errorTree(tree, "applyDynamicNamed does not support passing a vararg parameter")
+      else {
         def namedArgTuple(name: String, arg: untpd.Tree) = untpd.Tuple(List(Literal(Constant(name)), arg))
         def namedArgs = args.map {
           case NamedArg(argName, arg) => namedArgTuple(argName.toString, arg)
@@ -89,8 +89,7 @@ trait Dynamic { self: Typer with Applications =>
       case TypeApply(Select(qual, name), targs) if !isDynamicMethod(name) =>
         typedDynamicAssign(qual, name, targs)
       case _ =>
-        ctx.error("reassignment to val", tree.pos)
-        tree.withType(ErrorType)
+        errorTree(tree, "reassignment to val")
     }
   }
 
diff --git a/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala b/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
index a066fc04a..270ad6c8a 100644
--- a/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
+++ b/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
@@ -23,7 +23,7 @@ object ErrorReporting {
 
   def errorType(msg: => Message, pos: Position)(implicit ctx: Context): ErrorType = {
     ctx.error(msg, pos)
-    ErrorType
+    new ErrorType(msg)
   }
 
   def cyclicErrorMsg(ex: CyclicReference)(implicit ctx: Context) = {
diff --git a/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala b/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala
index ee2d68278..ca0af0e2b 100644
--- a/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala
+++ b/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala
@@ -189,9 +189,8 @@ trait TypeAssigner {
             val where = if (ctx.owner.exists) s" from ${ctx.owner.enclosingClass}" else ""
             val whyNot = new StringBuffer
             alts foreach (_.isAccessibleFrom(pre, superAccess, whyNot))
-            if (!tpe.isError)
-              ctx.error(ex"$what cannot be accessed as a member of $pre$where.$whyNot", pos)
-            ErrorType
+            if (tpe.isError) tpe
+            else errorType(ex"$what cannot be accessed as a member of $pre$where.$whyNot", pos)
           }
         }
         else if (d.symbol is TypeParamAccessor)
@@ -215,17 +214,17 @@ trait TypeAssigner {
     else if (site.derivesFrom(defn.DynamicClass) && !Dynamic.isDynamicMethod(name)) {
       TryDynamicCallType
     } else {
-      if (!site.isErroneous) {
+      if (site.isErroneous) UnspecifiedErrorType
+      else {
         def kind = if (name.isTypeName) "type" else "value"
         def addendum =
           if (site.derivesFrom(defn.DynamicClass)) "\npossible cause: maybe a wrong Dynamic method signature?"
           else ""
-        ctx.error(
+        errorType(
           if (name == nme.CONSTRUCTOR) ex"$site does not have a constructor"
           else NotAMember(site, name, kind),
           pos)
       }
-      ErrorType
     }
   }
 
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index eec3859f9..081e46841 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -351,10 +351,8 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     val ownType =
       if (rawType.exists)
         ensureAccessible(rawType, superAccess = false, tree.pos)
-      else {
-        error(new MissingIdent(tree, kind, name.show), tree.pos)
-        ErrorType
-      }
+      else
+        errorType(new MissingIdent(tree, kind, name.show), tree.pos)
 
     val tree1 = ownType match {
       case ownType: NamedType if !prefixIsElidable(ownType) =>
@@ -1965,10 +1963,25 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
       else err.typeMismatch(tree1, pt)
     }
 
+    /** If tree has an error type but no errors are reported yet, issue
+     *  the error message stored in the type.
+     *  One way this can happen is if implicit search causes symbols and types
+     *  to be completed. The types are stored by `typedAhead` so that they can be
+     *  retrieved later and thus avoid duplication of typechecking work.
+     *  But if the implicit search causing the `typedAhead` fails locally but
+     *  another alternative succeeds we can be left with an ErrorType in the
+     *  tree that went unreported. A scenario where this happens is i1802.scala.
+     */
+    def ensureReported(tp: Type) = tp match {
+      case err: ErrorType if !ctx.reporter.hasErrors => ctx.error(err.msg, tree.pos)
+      case _ =>
+    }
+
     tree match {
       case _: MemberDef | _: PackageDef | _: Import | _: WithoutTypeOrPos[_] => tree
       case _ => tree.tpe.widen match {
-        case _: ErrorType =>
+        case tp: FlexType =>
+          ensureReported(tp)
           tree
         case ref: TermRef =>
           pt match {