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.

1 files changed, 18 insertions, 5 deletions

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 {