SIL: Add forEachExpandedPackElement() and use it in type lowering

Probably there's no way to hit this right now, but there will be if
concrete formal PackTypes start showing up in AST.

Author: slavapestov

include/swift/SIL/AbstractionPattern.h

@@ -1395,7 +1395,7 @@ class AbstractionPattern {
 
   /// Is the given pack type a valid substitution of this abstraction
   /// pattern?
-  bool matchesPack(CanPackType substType);
+  bool matchesPack(CanPackType substType) const;
 
   bool isPack() const {
     switch (getKind()) {
@@ -1460,6 +1460,17 @@ class AbstractionPattern {
   void forEachPackElement(CanPackType substPackType,
          llvm::function_ref<void(PackElementGenerator &element)> fn) const;
 
+  /// Perform a parallel visitation of the elements of a pack type,
+  /// expanding the elements of the type.  This preserves the structure
+  /// of the *substituted* pack type: it will be called once per element
+  /// of the substituted type, in order.
+  ///
+  /// This pattern must match the substituted type, but it may be an
+  /// opaque pattern.
+  void forEachExpandedPackElement(CanPackType substPackType,
+      llvm::function_ref<void(AbstractionPattern origEltType,
+                              CanType substEltType)> handleElement) const;
+
   /// Given that the value being abstracted is a move only type, return the
   /// abstraction pattern with the move only bit removed.
   AbstractionPattern removingMoveOnlyWrapper() const;

lib/SIL/IR/AbstractionPattern.cpp

@@ -707,7 +707,7 @@ AbstractionPattern::getPackElementType(unsigned index) const {
   llvm_unreachable("bad kind");
 }
 
-bool AbstractionPattern::matchesPack(CanPackType substType) {
+bool AbstractionPattern::matchesPack(CanPackType substType) const {
   switch (getKind()) {
   case Kind::Invalid:
     llvm_unreachable("querying invalid abstraction pattern!");
@@ -750,11 +750,11 @@ void AbstractionPattern::forEachPackElement(CanPackType substType,
   elt.finish();
 }
 
-void AbstractionPattern::forEachExpandedPackElement(CanPackType substType,
+void AbstractionPattern::forEachExpandedPackElement(CanPackType substPackType,
                       llvm::function_ref<void(AbstractionPattern origEltType,
                                               CanType substEltType)>
                         handleElement) const {
-  assert(matchesPack(substType));
+  assert(matchesPack(substPackType));
 
   auto substEltTypes = substPackType.getElementTypes();
 

lib/SIL/IR/TypeLowering.cpp

@@ -2608,14 +2608,13 @@ static CanSILPackType computeLoweredPackType(TypeConverter &tc,
   SmallVector<CanType, 4> loweredElts;
   loweredElts.reserve(substType->getNumElements());
 
-  for (auto i : indices(substType->getElementTypes())) {
-    auto origEltType = origType.getPackElementType(i);
-    auto substEltType = substType.getElementType(i);
-
-    CanType loweredTy =
+  origType.forEachExpandedPackElement(substType,
+                                      [&](AbstractionPattern origEltType,
+                                          CanType substEltType) {
+    auto loweredTy =
         tc.getLoweredRValueType(context, origEltType, substEltType);
     loweredElts.push_back(loweredTy);
-  }
+  });
 
   bool elementIsAddress = true; // TODO
   SILPackType::ExtInfo extInfo(elementIsAddress);