Add a utility function for getting the interface value type of storage; NFC.

Because subscripts can be generic, this isn't quite as universally useful
as you might think; it's pretty much only useful when mapping types into
the context of the declaration, e.g. into an accessor.

Author: rjmccall

include/swift/AST/Decl.h

@@ -4134,6 +4134,9 @@ class AbstractStorageDecl : public ValueDecl {
   /// is a member.  Currently only variables can be static.
   inline bool isStatic() const; // defined in this header
 
+  /// \brief Return the interface type of the stored value.
+  Type getValueInterfaceType() const;
+
   /// \brief Determine how this storage is implemented.
   StorageImplInfo getImplInfo() const {
     if (auto ptr = Accessors.getPointer())

lib/AST/Decl.cpp

@@ -4071,6 +4071,12 @@ SourceLoc AbstractStorageDecl::getOverrideLoc() const {
   return SourceLoc();
 }
 
+Type AbstractStorageDecl::getValueInterfaceType() const {
+  if (auto var = dyn_cast<VarDecl>(this))
+    return var->getInterfaceType();
+  return cast<SubscriptDecl>(this)->getElementInterfaceType();
+}
+
 Type VarDecl::getType() const {
   if (!typeInContext) {
     const_cast<VarDecl *>(this)->typeInContext =

lib/ClangImporter/ImporterImpl.h

@@ -625,11 +625,9 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
     } else if (auto *CD = dyn_cast<ConstructorDecl>(decl)) {
       assert(CD->getInterfaceType());
       ty = CD->getResultInterfaceType();
-    } else if (auto *SD = dyn_cast<SubscriptDecl>(decl)) {
-      ty = SD->getElementInterfaceType();
     } else {
-      auto *VD = cast<VarDecl>(decl);
-      ty = VD->getInterfaceType()->getReferenceStorageReferent();
+      ty = cast<AbstractStorageDecl>(decl)->getValueInterfaceType()
+                                          ->getReferenceStorageReferent();
     }
 #endif
 

lib/Sema/TypeCheckDeclOverride.cpp

@@ -70,9 +70,7 @@ Type swift::getMemberTypeForComparison(ASTContext &ctx, ValueDecl *member,
 
   auto abstractStorage = dyn_cast<AbstractStorageDecl>(member);
   assert((method || abstractStorage) && "Not a method or abstractStorage?");
-  SubscriptDecl *subscript = nullptr;
-  if (abstractStorage)
-    subscript = dyn_cast<SubscriptDecl>(abstractStorage);
+  SubscriptDecl *subscript = dyn_cast_or_null<SubscriptDecl>(abstractStorage);
 
   auto memberType = member->getInterfaceType();
   if (derivedDecl) {