[TypeJoin] Replace getKind() with is<>(), and some other things.

Add a comment explaining that the distributive law isn't generally
true for lattices but should be for ours, and fix up the test case
to not use the real stdlib Strideable protocol in a couple places.

Author: rudkx

lib/AST/TypeJoinMeet.cpp

@@ -350,6 +350,14 @@ CanType TypeJoin::visitGenericFunctionType(CanType second) {
 //
 //   (A ^ B) v (C ^ D)
 // = (A v C) ^ (A v D) ^ (B v C) ^ (B v D)
+//
+// In general this law only applies to distributive lattices.
+//
+// In our case, this should be safe because our meet operation only
+// produces an existing nominal type when it is one of the operands of
+// the operation. So we can never arbitrarily climb down the lattice
+// in ways that would break distributivity.
+//
 CanType TypeJoin::computeProtocolCompositionJoin(ArrayRef<Type> firstMembers,
                                                  ArrayRef<Type> secondMembers) {
   SmallVector<Type, 8> result;
@@ -388,13 +396,13 @@ CanType TypeJoin::visitProtocolCompositionType(CanType second) {
   assert(First != second);
 
   // FIXME: Handle other types here.
-  if (First->getKind() != TypeKind::Protocol &&
-      First->getKind() != TypeKind::ProtocolComposition)
+  if (!First->is<ProtocolType>() &&
+      !First->is<ProtocolCompositionType>())
     return TheAnyType;
 
   SmallVector<Type, 1> protocolType;
   ArrayRef<Type> firstMembers;
-  if (First->getKind() == TypeKind::Protocol) {
+  if (First->is<ProtocolType>()) {
     protocolType.push_back(First);
     firstMembers = protocolType;
   } else {
@@ -434,8 +442,8 @@ static bool isSupertypeOf(ProtocolDecl *super, ProtocolDecl *sub) {
 CanType TypeJoin::visitProtocolType(CanType second) {
   assert(First != second);
 
-  assert(First->getKind() != TypeKind::ProtocolComposition &&
-         second->getKind() != TypeKind::ProtocolComposition);
+  assert(!First->is<ProtocolCompositionType>() &&
+         !second->is<ProtocolCompositionType>());
 
   // FIXME: Handle other types here.
   if (First->getKind() != second->getKind())

test/Sema/type_join.swift

@@ -75,9 +75,9 @@ expectEqualType(Builtin.type_join(FakeEquatable.self, FakeEquatable.self), FakeE
 expectEqualType(Builtin.type_join(FakeHashable.self, FakeEquatable.self), FakeEquatable.self)
 expectEqualType(Builtin.type_join(FakeEquatable.self, FakeHashable.self), FakeEquatable.self)
 expectEqualType(Builtin.type_join(FakeNumeric.self, FakeHashable.self), FakeEquatable.self)
-expectEqualType(Builtin.type_join((FakeHashable & Strideable).self, (FakeHashable & FakeNumeric).self),
+expectEqualType(Builtin.type_join((FakeHashable & FakeStrideable).self, (FakeHashable & FakeNumeric).self),
                                   FakeHashable.self)
-expectEqualType(Builtin.type_join((FakeNumeric & Strideable).self,
+expectEqualType(Builtin.type_join((FakeNumeric & FakeStrideable).self,
                                   (FakeHashable & FakeNumeric).self), FakeNumeric.self)
 expectEqualType(Builtin.type_join(FakeBinaryInteger.self, FakeFloatingPoint.self),
                                   (FakeHashable & FakeNumeric & FakeStrideable).self)