[CSSimplify] Delay binding metatype instance types if one side is type variable

While matching two metatypes where one side is a class (which could have
supertypes) and another is a type variable use `subtype` constraint to
delay binding types together because there is a real possibility that
there exists a superclass associated with yet free type variable which
would be a better match when attempted.

Resolves: rdar://problem/44816848

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -1962,21 +1962,23 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
       auto meta1 = cast<AnyMetatypeType>(desugar1);
       auto meta2 = cast<AnyMetatypeType>(desugar2);
 
-      ConstraintKind subKind = ConstraintKind::Equal;
       // A.Type < B.Type if A < B and both A and B are classes.
-      if (isa<MetatypeType>(meta1) &&
-          meta1->getInstanceType()->mayHaveSuperclass() &&
-          meta2->getInstanceType()->getClassOrBoundGenericClass())
-        subKind = std::min(kind, ConstraintKind::Subtype);
       // P.Type < Q.Type if P < Q, both P and Q are protocols, and P.Type
-      // and Q.Type are both existential metatypes.
-      else if (isa<ExistentialMetatypeType>(meta1))
-        subKind = std::min(kind, ConstraintKind::Subtype);
-      
-      return matchTypes(meta1->getInstanceType(), meta2->getInstanceType(),
-                        subKind, subflags,
-                        locator.withPathElement(
-                          ConstraintLocator::InstanceType));
+      // 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::Equal;
+      }
+
+      return matchTypes(
+          instanceType1, instanceType2, subKind, subflags,
+          locator.withPathElement(ConstraintLocator::InstanceType));
     }
 
     case TypeKind::Function: {

test/Constraints/rdar44816848.swift

@@ -0,0 +1,16 @@
+// RUN: %target-typecheck-verify-swift
+
+class A {}
+class B: A {}
+class C: A {}
+
+struct S {
+  func foo<T: A>(types: [T.Type]) {}
+}
+
+func bar(_ s: S, _ forced_s: S!) {
+  s.foo(types: [A.self, B.self]) // ok
+  s.foo(types: [B.self, A.self]) // ok
+  forced_s.foo(types: [A.self, B.self]) // ok
+  forced_s.foo(types: [B.self, A.self]) // ok
+}