Fix dotc bootstrap failure

During an attempted dotty bootstrap it was noted that Types.scala did not compile
anymore, because `checkUnique` threw a `TypeError` during erasure. The issue was an
overloaded member `name` in TermrefWithSig. In NamedType:

    def name: Name

In TermRef:

    def name: TermName

Before erasure, there's one member `name`, after erasure there are two (because after
erasure result type counts). The error arose when trying to recompute a member
of a `TermRefWithSig` where the name is `name` and the expected signature is `(Nil, ?)`.
Since there are two members that match the name and the signature, `checkUnique`
triggered a `TypeError`. Before adding `checkUnique`, the previous `atSignature`
call would just have returned an arbitrary choice among the two alternative definitions
of `name`.

The fix is not to use `checkUnique` but to fall back to `d.current` in the case where
several alternatives appear.

Interestingly, the failure only triggers when -Ycheck options are *disabled*. I added a new
test that compiles Types.scala without checks, so we catch this and possibly  similar bugs
in the future.

Author: odersky

src/dotty/tools/dotc/core/Types.scala

@@ -1437,7 +1437,11 @@ object Types {
       asMemberOf(prefix) match {
         case NoDenotation => d.current
         case newd: SingleDenotation => newd
-        case newd => newd.atSignature(d.signature).checkUnique.orElse(d.current)
+        case newd =>
+          newd.atSignature(d.signature) match {
+            case newd1: SingleDenotation if newd1.exists => newd1
+            case _ => d.current
+          }
       }
 
     private def denotOfSym(sym: Symbol)(implicit ctx: Context): Denotation = {

test/dotc/tests.scala

@@ -157,6 +157,7 @@ class tests extends CompilerTest {
   @Test def dotc_ast = compileDir(dotcDir, "ast")
   @Test def dotc_config = compileDir(dotcDir, "config")
   @Test def dotc_core = compileDir(dotcDir, "core")("-Yno-double-bindings" :: allowDeepSubtypes)// twice omitted to make tests run faster
+  @Test def dotc_core_Types = compileFile(dotcDir, "core/Types")(noCheckOptions)
 
 // This directory doesn't exist anymore
 //  @Test def dotc_core_pickling = compileDir(coreDir, "pickling")(allowDeepSubtypes)// twice omitted to make tests run faster