[cxx-interop] Empty base classes have zero offset.

Make sure empty base classes don't affect the offset of stored properties.

Author: zoecarver

lib/IRGen/GenStruct.cpp

@@ -460,8 +460,14 @@ namespace {
 
       unsigned baseOffset = 0;
       if (auto cxxRecord = dyn_cast<clang::CXXRecordDecl>(ClangDecl)) {
-        baseOffset =
-            std::distance(cxxRecord->bases().begin(), cxxRecord->bases().end());
+        baseOffset = llvm::count_if(cxxRecord->bases(), [](auto base) {
+          auto baseType = base.getType().getCanonicalType();
+
+          auto baseRecord = cast<clang::RecordType>(baseType)->getDecl();
+          auto baseCxxRecord = cast<clang::CXXRecordDecl>(baseRecord);
+
+          return !baseCxxRecord->isEmpty();
+        });
       }
 
       // Otherwise, project from the base.

test/Interop/Cxx/class/inheritance/Inputs/functions.h

@@ -49,3 +49,12 @@ struct DerivedFromDerived : Derived {
 };
 
 struct DerivedFromNonTrivial : NonTrivial {};
+
+struct EmptyBaseClass {
+  const char *inBase() const { return "EmptyBaseClass::inBase"; }
+};
+
+struct DerivedFromEmptyBaseClass : EmptyBaseClass {
+  int a = 42;
+  int b = 42;
+};

test/Interop/Cxx/class/inheritance/functions.swift

@@ -69,4 +69,10 @@ FunctionsTestSuite.test("base member from derived from non trivial") {
   expectEqual(String(cString: dnt.inNonTrivialWithArgs(0, 1)!), "NonTrivial::inNonTrivialWithArgs")
 }
 
+FunctionsTestSuite.test("non-empty derived from empty class") {
+  let derived = DerivedFromEmptyBaseClass()
+  expectEqual(String(cString: derived.inBase()!), "EmptyBaseClass::inBase")
+  expectEqual(derived.b, 42)
+}
+
 runAllTests()