Handle complex context merging cases

Test case in isApplicableSafe.scala. It turns out that this
requires a context merge using the new `&' operator. Sequence of actions:

 1) Typecheck argument in typerstate 1.
 2) Cache argument.
 3) Evolve same typer state (to typecheck other arguments, say)
    leading to a different constraint.
 4) Take typechecked argument in same state.

It turns out that the merge in TyperState is needed not just for
isApplicableSafe but also for (e.g. erased-lubs.scala) as well as
many parts of dotty itself.

Author: odersky

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

@@ -96,7 +96,8 @@ extends TyperState(r) {
 
   override def reporter = myReporter
 
-  private var myConstraint: Constraint = previous.constraint
+  private val previousConstraint = previous.constraint
+  private var myConstraint: Constraint = previousConstraint
 
   override def constraint = myConstraint
   override def constraint_=(c: Constraint)(implicit ctx: Context) = {
@@ -129,8 +130,9 @@ extends TyperState(r) {
   override def commit()(implicit ctx: Context) = {
     val targetState = ctx.typerState
     assert(isCommittable)
-    if (targetState eq previous) targetState.constraint = constraint
-    else targetState.constraint &= constraint
+    targetState.constraint =
+      if (targetState.constraint eq previousConstraint) constraint
+      else targetState.constraint & constraint
     constraint foreachTypeVar { tvar =>
       if (tvar.owningState eq this)
         tvar.owningState = targetState

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

@@ -78,7 +78,8 @@ object Inferencing {
     def apply(x: Boolean, tp: Type): Boolean = tp.dealias match {
       case _: WildcardType | _: ProtoType =>
         false
-      case tvar: TypeVar if !tvar.isInstantiated =>
+      case tvar: TypeVar
+      if !tvar.isInstantiated && ctx.typerState.constraint.contains(tvar) =>
         force.appliesTo(tvar) && {
           val direction = instDirection(tvar.origin)
           if (direction != 0) {