Revert "[ConstraintSolver] Prioritize certain type variables while looking for bindings"

This reverts commit 001b097.

Author: xedin

lib/Sema/CSBindings.cpp

@@ -361,10 +361,6 @@ ConstraintSystem::getPotentialBindings(TypeVariableType *typeVar) {
                ConstraintClassification::Relational &&
            "only relational constraints handled here");
 
-    // Record constraint which contributes to the
-    // finding of pontential bindings.
-    result.Sources.insert(constraint);
-
     auto first = simplifyType(constraint->getFirstType());
     auto second = simplifyType(constraint->getSecondType());
 

lib/Sema/CSSimplify.cpp

@@ -1695,14 +1695,13 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
         }
 
         // If we have a binding for the right-hand side
-        // (argument type used in the body) don't try
-        // to bind it to the left-hand side (parameter type)
-        // directly, because there could be an implicit
-        // conversion between them, and actual binding
-        // can only come from the left-hand side.
+        // (argument type) don't try to bind it to the left-hand
+        // side (parameter type) directly, because their
+        // relationship is contravariant and the actual
+        // binding can only come from the left-hand side.
         addUnsolvedConstraint(
-            Constraint::create(*this, ConstraintKind::Equal, typeVar1, type2,
-                               getConstraintLocator(locator)));
+            Constraint::create(*this, ConstraintKind::ArgumentConversion, type2,
+                               typeVar1, getConstraintLocator(locator)));
         return SolutionKind::Solved;
       }
 

lib/Sema/ConstraintSystem.h

@@ -33,7 +33,6 @@
 #include "swift/AST/TypeCheckerDebugConsumer.h"
 #include "llvm/ADT/ilist.h"
 #include "llvm/ADT/PointerUnion.h"
-#include "llvm/ADT/SetOperations.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/Support/ErrorHandling.h"
@@ -2578,9 +2577,6 @@ class ConstraintSystem {
     /// Tracks the position of the last known supertype in the group.
     Optional<unsigned> lastSupertypeIndex;
 
-    /// A set of all constraints which contribute to pontential bindings.
-    llvm::SmallPtrSet<Constraint *, 8> Sources;
-
     PotentialBindings(TypeVariableType *typeVar) : TypeVar(typeVar) {}
 
     /// Determine whether the set of bindings is non-empty.
@@ -2604,29 +2600,7 @@ class ConstraintSystem {
     /// \c x is a better set of bindings that \c y.
     friend bool operator<(const PotentialBindings &x,
                           const PotentialBindings &y) {
-      if (formBindingScore(x) < formBindingScore(y))
-        return true;
-
-      if (!x.hasNonDefaultableBindings())
-        return false;
-
-      llvm::SmallPtrSet<Constraint *, 8> intersection(x.Sources);
-      llvm::set_intersect(intersection, y.Sources);
-
-      // Some relational constraints dictate certain
-      // ordering when it comes to attempting binding
-      // of type variables, where left-hand side is
-      // always more preferrable than right-hand side.
-      for (const auto *constraint : intersection) {
-        if (constraint->getKind() != ConstraintKind::Subtype)
-          continue;
-
-        auto lhs = constraint->getFirstType();
-        if (auto *typeVar = lhs->getAs<TypeVariableType>())
-          return x.TypeVar == typeVar;
-      }
-
-      return false;
+      return formBindingScore(x) < formBindingScore(y);
     }
 
     void foundLiteralBinding(ProtocolDecl *proto) {

test/Constraints/generics.swift

@@ -504,27 +504,3 @@ func rdar27700622<E: Comparable>(_ input: [E]) -> [E] {
 
   return rdar27700622(lhs) + [pivot] + rdar27700622(rhs) // Ok
 }
-
-// rdar://problem/22898292 - Type inference failure with constrained subclass
-protocol P_22898292 {}
-
-do {
-  func construct_generic<T: P_22898292>(_ construct: () -> T) -> T { return construct() }
-
-  class A {}
-  class B : A, P_22898292 {}
-
-  func foo() -> B { return B() }
-  func bar(_ value: A) {}
-  func baz<T: A>(_ value: T) {}
-
-  func rdar_22898292_1() {
-    let x = construct_generic { foo() } // returns A
-    bar(x) // Ok
-    bar(construct_generic { foo() }) // Ok
-  }
-
-  func rdar22898292_2<T: B>(_ d: T) {
-    _ = { baz($0) }(construct_generic { d }) // Ok
-  }
-}