[CS] Account for type variables when matching metatypes

Previously we could prematurely attempt to perform
a bind of class metatypes without checking for
subtyping. Tweak the logic to not perform a bind
if we can't prove that we're dealing with non-class
types.

Author: hamishknight

lib/Sema/CSSimplify.cpp

@@ -4373,22 +4373,34 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
       auto meta1 = cast<AnyMetatypeType>(desugar1);
       auto meta2 = cast<AnyMetatypeType>(desugar2);
 
-      // A.Type < B.Type if A < B and both A and B are classes.
-      // P.Type < Q.Type if P < Q, both P and Q are protocols, and P.Type
-      // and Q.Type are both existential metatypes
-      auto subKind = std::min(kind, ConstraintKind::Subtype);
-      // If instance types can't have a subtype relationship
-      // it means that such types can be simply equated.
       auto instanceType1 = meta1->getInstanceType();
       auto instanceType2 = meta2->getInstanceType();
-      if (isa<MetatypeType>(meta1) &&
-          !(instanceType1->mayHaveSuperclass() &&
-            instanceType2->getClassOrBoundGenericClass())) {
-        subKind = ConstraintKind::Bind;
-      }
+
+      // A.Type < B.Type if A < B and both A and B are classes.
+      // P.Type < Q.Type if P < Q, both P and Q are protocols, and P.Type
+      // and Q.Type are both existential metatypes.
+      auto getSubKind = [&]() -> ConstraintKind {
+        auto subKind = std::min(kind, ConstraintKind::Subtype);
+
+        // If we have existential metatypes, we need to perform subtyping.
+        if (!isa<MetatypeType>(meta1))
+          return subKind;
+
+        // If the LHS cannot be a type with a superclass, we can perform a bind.
+        if (!instanceType1->isTypeVariableOrMember() &&
+            !instanceType1->mayHaveSuperclass())
+          return ConstraintKind::Bind;
+
+        // If the RHS cannot be a class type, we can perform a bind.
+        if (!instanceType2->isTypeVariableOrMember() &&
+            !instanceType2->getClassOrBoundGenericClass())
+          return ConstraintKind::Bind;
+
+        return subKind;
+      };
 
       auto result =
-          matchTypes(instanceType1, instanceType2, subKind, subflags,
+          matchTypes(instanceType1, instanceType2, getSubKind(), subflags,
                      locator.withPathElement(ConstraintLocator::InstanceType));
 
       // If matching of the instance types resulted in the failure make sure