Avoid propagating unresolved implicits

When an implicit argument is not found, we should in any case
assume the result type of the implicit method as the type of
the tree (after reporting an error, of course). If we don't
do that, we get implicit errors on weird positions when we try
to find an implicit argument for the same tree again. This
caused a spurious error in subtyping.scala, and also caused
an additional error at the end of EqualityStrawman1.scala.

Author: odersky

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

@@ -1529,7 +1529,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
           }
           def issueErrors() = {
             for (err <- errors) ctx.error(err(), tree.pos.endPos)
-            tree
+            tree.withType(wtp.resultType)
           }
           val args = (wtp.paramNames, wtp.paramTypes).zipped map { (pname, formal) =>
             def where = d"parameter $pname of $methodStr"

tests/neg/subtyping.scala

@@ -6,7 +6,7 @@ class A extends B
 object Test {
   def test1(): Unit = {
     implicitly[B#X <:< A#X] // error: no implicit argument
-  } // error: no implicit argument
+  }
   def test2(): Unit = {
     val a : { type T; type U } = ??? // error // error
     implicitly[a.T <:< a.U] // error: no implicit argument