[CSBindings] Infer any type variable connected to a code completion token as a potential hole

If type variable is associated with a code completion token
it's possible that it doesn't have enough contextual information
to be resolved to anything, so let's add a hole type which originates
from type variable associated with code completion expression to
make this relationship explicit and avoid "fixing" problems rooted
in fact that type variable is underconstrained due to code completion.

Author: xedin

lib/Sema/CSBindings.cpp

@@ -377,6 +377,15 @@ void ConstraintSystem::PotentialBindings::finalize(
       PotentiallyIncomplete = true;
     }
 
+    // If this type variable is associated with a code completion token
+    // and it failed to infer any bindings let's adjust hole's locator
+    // to point to a code completion token to avoid attempting to "fix"
+    // this problem since its rooted in the fact that constraint system
+    // is under-constrained.
+    if (AssociatedCodeCompletionToken) {
+      locator = cs.getConstraintLocator(AssociatedCodeCompletionToken);
+    }
+
     addPotentialBinding(PotentialBinding::forHole(TypeVar, locator));
   }
 
@@ -768,6 +777,18 @@ ConstraintSystem::getPotentialBindingForRelationalConstraint(
 
     result.InvolvesTypeVariables = true;
 
+    // If current type variable is associated with a code completion token
+    // it's possible that it doesn't have enough contextual information
+    // to be resolved to anything, so let's note that fact in the potential
+    // bindings and use it when forming a hole if there are no other bindings
+    // available.
+    if (auto *locator = bindingTypeVar->getImpl().getLocator()) {
+      if (locator->directlyAt<CodeCompletionExpr>()) {
+        result.AssociatedCodeCompletionToken = locator->getAnchor();
+        result.PotentiallyIncomplete = true;
+      }
+    }
+
     if (constraint->getKind() == ConstraintKind::Subtype &&
         kind == AllowedBindingKind::Subtypes) {
       result.SubtypeOf.insert(bindingTypeVar);

lib/Sema/ConstraintSystem.h

@@ -4644,6 +4644,8 @@ class ConstraintSystem {
     /// `bind param` are present in the system.
     bool PotentiallyIncomplete = false;
 
+    ASTNode AssociatedCodeCompletionToken = ASTNode();
+
     /// Whether this type variable has literal bindings.
     LiteralBindingKind LiteralBinding = LiteralBindingKind::None;