[cxx-interop] Fix infinite recursion of PackExpansion type.

Fix infinite recursion issue in the SwiftTypeConverter. This patch is
purely to prevent crashes.

Author: zoecarver

lib/ClangImporter/ImportType.cpp

@@ -830,23 +830,32 @@ namespace {
       return { mappedType, underlyingResult.Hint };
     }
 
-#define SUGAR_TYPE(KIND)                                            \
-    ImportResult Visit##KIND##Type(const clang::KIND##Type *type) { \
-      return Visit(type->desugar());                                \
-    }
-    SUGAR_TYPE(TypeOfExpr)
-    SUGAR_TYPE(TypeOf)
-    SUGAR_TYPE(Decltype)
-    SUGAR_TYPE(UnaryTransform)
-    SUGAR_TYPE(Elaborated)
-    SUGAR_TYPE(SubstTemplateTypeParm)
-    SUGAR_TYPE(TemplateSpecialization)
-    SUGAR_TYPE(Auto)
-    SUGAR_TYPE(DeducedTemplateSpecialization)
-    SUGAR_TYPE(Adjusted)
-    SUGAR_TYPE(PackExpansion)
-    SUGAR_TYPE(Attributed)
+    // TODO: add custom visitors for these types.
+#define MAYBE_SUGAR_TYPE(KIND)                                                 \
+    ImportResult Visit##KIND##Type(const clang::KIND##Type *type) {            \
+      if (type->isSugared())                                                   \
+        return Visit(type->desugar());                                         \
+      return Type();                                                           \
+    }
+    MAYBE_SUGAR_TYPE(TypeOfExpr)
+    MAYBE_SUGAR_TYPE(TypeOf)
+    MAYBE_SUGAR_TYPE(Decltype)
+    MAYBE_SUGAR_TYPE(UnaryTransform)
+    MAYBE_SUGAR_TYPE(TemplateSpecialization)
+    MAYBE_SUGAR_TYPE(Auto)
+    MAYBE_SUGAR_TYPE(DeducedTemplateSpecialization)
+    MAYBE_SUGAR_TYPE(PackExpansion)
+
+    // These types are ALWAYS sugared.
+#define SUGAR_TYPE(KIND)                                                       \
+    ImportResult Visit##KIND##Type(const clang::KIND##Type *type) {            \
+      return Visit(type->desugar());                                           \
+    }
     SUGAR_TYPE(MacroQualified)
+    SUGAR_TYPE(Attributed)
+    SUGAR_TYPE(Adjusted)
+    SUGAR_TYPE(SubstTemplateTypeParm)
+    SUGAR_TYPE(Elaborated)
 
     ImportResult VisitDecayedType(const clang::DecayedType *type) {
       clang::ASTContext &clangCtx = Impl.getClangASTContext();

test/Interop/Cxx/templates/Inputs/function-templates.h

@@ -20,3 +20,22 @@ template <class R, class T, class U> R returns_template(T a, U b) {
 
 // Same here:
 template <class T> void cannot_infer_template() {}
+
+// TODO: We should support these types. Until then, make sure we don't crash when importing.
+template<class... Ts>
+void testPackExpansion(Ts...) { }
+
+template<class T>
+void testTypeOfExpr(T a, typeof(a + 1) b) { }
+
+template<class T>
+void testTypeOf(T a, typeof a b) { }
+
+template<class T>
+decltype(auto) testAuto(T arg) {
+  return arg;
+}
+
+// TODO: Add tests for Decltype, UnaryTransform, and TemplateSpecialization with a dependent type once those are supported.
+
+// TODO: Add test for DeducedTemplateSpecializationType once we support class templates.