[ConstraintSystem] NFC: Rename isFullyBound -> isDelayed and clarify associated comment

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -4762,9 +4762,16 @@ class ConstraintSystem {
     /// Determine whether the set of bindings is non-empty.
     explicit operator bool() const { return !Bindings.empty(); }
 
-    /// Whether these bindings should be delayed until the rest of the
-    /// constraint system is considered "fully bound".
-    bool isFullyBound() const;
+    /// Determine whether attempting this type variable should be
+    /// delayed until the rest of the constraint system is considered
+    /// "fully bound" meaning constraints, which affect completeness
+    /// of the binding set, for this type variable such as - member
+    /// constraint, disjunction, function application etc. - are simplified.
+    ///
+    /// Note that in some situations i.e. when there are no more
+    /// disjunctions or type variables left to attempt, it's still
+    /// okay to attempt "delayed" type variable to make forward progress.
+    bool isDelayed() const;
 
     /// Whether the bindings represent (potentially) incomplete set,
     /// there is no way to say with absolute certainty if that's the
@@ -4808,7 +4815,7 @@ class ConstraintSystem {
 
       return std::make_tuple(b.isHole(),
                              !hasNoDefaultableBindings,
-                             b.isFullyBound(),
+                             b.isDelayed(),
                              b.isSubtypeOfExistentialType(),
                              b.InvolvesTypeVariables,
                              static_cast<unsigned char>(b.LiteralBinding),
@@ -4955,8 +4962,8 @@ class ConstraintSystem {
       out.indent(indent);
       if (isPotentiallyIncomplete())
         out << "potentially_incomplete ";
-      if (isFullyBound())
-        out << "fully_bound ";
+      if (isDelayed())
+        out << "delayed ";
       if (isSubtypeOfExistentialType())
         out << "subtype_of_existential ";
       if (LiteralBinding != LiteralBindingKind::None)

lib/Sema/CSBindings.cpp

@@ -33,7 +33,7 @@ bool ConstraintSystem::PotentialBinding::isViableForJoin() const {
          !isDefaultableBinding();
 }
 
-bool ConstraintSystem::PotentialBindings::isFullyBound() const {
+bool ConstraintSystem::PotentialBindings::isDelayed() const {
   if (!DelayedBy.empty())
     return true;
 
@@ -788,7 +788,7 @@ bool ConstraintSystem::PotentialBindings::isViable(
 
 bool ConstraintSystem::PotentialBindings::favoredOverDisjunction(
     Constraint *disjunction) const {
-  if (isHole() || isFullyBound())
+  if (isHole() || isDelayed())
     return false;
 
   // If this bindings are for a closure and there are no holes,