[ConstraintSystem] Allow code completion type to be a hole

Code completion expression type could be a hole if and only
if there is obosolutely no contextual information available
e.g. `let _ = .#^MEMBER^#`

Author: xedin

lib/Sema/CSBindings.cpp

@@ -335,6 +335,14 @@ void ConstraintSystem::PotentialBindings::finalize(
     if (locator->isLastElement<LocatorPathElt::MemberRefBase>())
       PotentiallyIncomplete = true;
 
+    // Delay resolution of the code completion expression until
+    // the very end to give it a chance to be bound to some
+    // contextual type even if it's a hole.
+    if (locator->directlyAt<CodeCompletionExpr>()) {
+      FullyBound = true;
+      PotentiallyIncomplete = true;
+    }
+
     addPotentialBinding(PotentialBinding::forHole(TypeVar, locator));
   }
 

lib/Sema/CSGen.cpp

@@ -1009,23 +1009,9 @@ namespace {
     virtual Type visitCodeCompletionExpr(CodeCompletionExpr *E) {
       CS.Options |= ConstraintSystemFlags::SuppressDiagnostics;
       auto locator = CS.getConstraintLocator(E);
-      auto ty = CS.createTypeVariable(locator,
-                                      TVO_CanBindToLValue |
-                                      TVO_CanBindToNoEscape);
-
-      // Defaults to the type of the base expression if we have a base
-      // expression.
-      // FIXME: This is just to keep the old behavior where `foo(base.<HERE>)`
-      // the argument is type checked to the type of the 'base'. Ideally, code
-      // completion expression should be defauled to 'UnresolvedType'
-      // regardless of the existence of the base expression. But the constraint
-      // system is simply not ready for that.
-      if (auto base = E->getBase()) {
-        CS.addConstraint(ConstraintKind::Defaultable, ty, CS.getType(base),
-                         locator);
-      }
-
-      return ty;
+      return CS.createTypeVariable(locator, TVO_CanBindToLValue |
+                                                TVO_CanBindToNoEscape |
+                                                TVO_CanBindToHole);
     }
 
     Type visitNilLiteralExpr(NilLiteralExpr *expr) {