Change treatment of signatures in TermRefs

 - don't compute members when unpickling termrefs, it caused
   cyclic reference errors on tasty-bootstrap.

 - to make this robust, have currentSignature in TermRef return
   a NotAMethof if not symbol or denotation is known.

Author: odersky

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

@@ -1535,6 +1535,10 @@ object Types {
       else symbol.asSeenFrom(prefix).signature
     }
 
+    /** The signature of the current denotation if it is known without forcing.
+     *  Otherwise the signature of the current symbol if it is known without forcing.
+     *  Otherwise NotAMethod.
+     */
     private def currentSignature(implicit ctx: Context): Signature =
       if (mySig != null) mySig
       else {
@@ -1543,7 +1547,7 @@ object Types {
         else {
           val sym = currentSymbol
           if (sym.exists) sym.asSeenFrom(prefix).signature
-          else null
+          else Signature.NotAMethod
         }
       }
 

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -296,22 +296,16 @@ class TreeUnpickler(reader: TastyReader, nameAtRef: NameRef => TermName, posUnpi
           readPackageRef().termRef
         case TYPEREF =>
           val name = readName().toTypeName
-          if (Config.newScheme) {
-            val prefix = readType()
-            TypeRef(prefix, name, prefix.member(name))
-          }
-          else TypeRef(readType(), name)
+          TypeRef(readType(), name)
         case TERMREF =>
           val sname = readName()
           val prefix = readType()
-          if (Config.newScheme)
-            sname match {
-              case SignedName(name, sig) =>
-                TermRef(prefix, name, prefix.member(name).atSignature(sig))
-              case name =>
-                TermRef(prefix, name, prefix.member(name))
-            }
-          else TermRef(prefix, sname)
+          sname match {
+            case SignedName(name, sig) =>
+              TermRef(prefix, name, prefix.member(name).atSignature(sig))
+            case name =>
+              TermRef(prefix, name)
+          }
         case THIS =>
           ThisType.raw(readType().asInstanceOf[TypeRef])
         case RECtype =>