Sema: Collect defaults in PotentialBindings::infer()

Author: slavapestov

include/swift/Sema/CSBindings.h

@@ -248,6 +248,9 @@ struct PotentialBindings {
   /// The set of protocol conformance requirements imposed on this type variable.
   llvm::SmallVector<Constraint *, 4> Protocols;
 
+  /// The set of fallback constraints imposed on this type variable.
+  llvm::SmallVector<Constraint *, 2> Defaults;
+
   ASTNode AssociatedCodeCompletionToken = ASTNode();
 
   /// Add a potential binding to the list of bindings,

lib/Sema/CSBindings.cpp

@@ -61,18 +61,17 @@ BindingSet::BindingSet(ConstraintSystem &CS, TypeVariableType *TypeVar,
       addLiteralRequirement(constraint);
       break;
 
-    case ConstraintKind::Defaultable:
-    case ConstraintKind::FallbackType:
-      // Do these in a separate pass.
-      if (isDirectRequirement(CS, TypeVar, constraint))
-        addDefault(constraint);
-      break;
-
     default:
       break;
     }
   }
 
+  for (auto *constraint : info.Defaults) {
+    // Do these in a separate pass.
+    if (isDirectRequirement(CS, TypeVar, constraint))
+      addDefault(constraint);
+  }
+
   for (auto &entry : info.AdjacentVars)
     AdjacentVars.insert(entry.first);
 }
@@ -2085,9 +2084,12 @@ void PotentialBindings::infer(ConstraintSystem &CS,
   case ConstraintKind::SameShape:
   case ConstraintKind::MaterializePackExpansion:
   case ConstraintKind::LiteralConformsTo:
+    // Constraints from which we can't do anything.
+    break;
+
   case ConstraintKind::Defaultable:
   case ConstraintKind::FallbackType:
-    // Constraints from which we can't do anything.
+    Defaults.push_back(constraint);
     break;
 
   // For now let's avoid inferring protocol requirements from