Fix #7750: Circumvent an assertion failure for erroneous code

Author: odersky

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

@@ -716,7 +716,7 @@ trait Checking {
   def checkImplicitConversionUseOK(sym: Symbol, posd: Positioned)(implicit ctx: Context): Unit =
     if (sym.exists) {
       val conv =
-        if (sym.isOneOf(GivenOrImplicit)) sym
+        if (sym.isOneOf(GivenOrImplicit) || sym.info.isErroneous) sym
         else {
           assert(sym.name == nme.apply)
           sym.owner

tests/neg/i7750.scala

@@ -0,0 +1,3 @@
+package object A extends runtime.A {
+    implicit def a( : ) = 1() // error // error // error
+}