Survive class of errors in implicit search

An `C.this` term with erroneous class `C` will get a
`NoPrefix` type. A subsequent implicit search on this
crashed. This is fixed now.

Fixes #324.

Author: odersky

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

@@ -402,7 +402,8 @@ trait Implicits { self: Typer =>
         || (to isRef defn.ObjectClass)
         || (to isRef defn.UnitClass)
         || (from.tpe isRef defn.NothingClass)
-        || (from.tpe isRef defn.NullClass)) NoImplicitMatches
+        || (from.tpe isRef defn.NullClass)
+        || (from.tpe eq NoPrefix)) NoImplicitMatches
     else
       try inferImplicit(to.stripTypeVar.widenExpr, from, from.pos)
       catch {

test/dotc/tests.scala

@@ -155,6 +155,7 @@ class tests extends CompilerTest {
   @Test def neg_i0091_infpaths = compileFile(negDir, "i0091-infpaths", xerrors = 3)
   @Test def neg_i0248_inherit_refined = compileFile(negDir, "i0248-inherit-refined", xerrors = 4)
   @Test def neg_i0281 = compileFile(negDir, "i0281-null-primitive-conforms", xerrors = 3)
+  @Test def neg_i324 = compileFile(negDir, "i324", xerrors = 2)
   @Test def neg_i583 = compileFile(negDir, "i0583-skolemize", xerrors = 2)
   @Test def neg_i941 = compileFile(negDir, "i941", xerrors = 3)
   @Test def neg_finalSealed = compileFile(negDir, "final-sealed", xerrors = 2)

tests/neg/i324.scala

@@ -0,0 +1,5 @@
+class O
+object O {
+   val x: this.type = OO.this
+   val y: O = OO.this
+}