[DSLX:FE] Fix slice cloning.

Author: cdleary

xls/dslx/frontend/ast_cloner.cc

@@ -633,9 +633,16 @@ class AstCloner : public AstNodeVisitor {
 
   absl::Status HandleSlice(const Slice* n) override {
     XLS_RETURN_IF_ERROR(VisitChildren(n));
-    old_to_new_[n] = module_->Make<Slice>(
-        n->GetSpan().value(), down_cast<Expr*>(old_to_new_.at(n->start())),
-        down_cast<Expr*>(old_to_new_.at(n->limit())));
+
+    Expr* new_start = n->start() == nullptr
+                          ? nullptr
+                          : down_cast<Expr*>(old_to_new_.at(n->start()));
+    Expr* new_limit = n->limit() == nullptr
+                          ? nullptr
+                          : down_cast<Expr*>(old_to_new_.at(n->limit()));
+
+    old_to_new_[n] =
+        module_->Make<Slice>(n->GetSpan().value(), new_start, new_limit);
     return absl::OkStatus();
   }
 

xls/dslx/frontend/ast_cloner_test.cc

@@ -511,6 +511,22 @@ TEST(AstClonerTest, TypeAlias) {
   XLS_ASSERT_OK(VerifyClone(type_alias, clone, file_table));
 }
 
+// This is an interesting case because the start and limit of the slice are
+// nullptr.
+TEST(AstClonerTest, SliceWithNullptrs) {
+  constexpr std::string_view kProgram =
+      R"(const MOL = u32:42; const MOL2 = MOL[:];)";
+
+  FileTable file_table;
+  XLS_ASSERT_OK_AND_ASSIGN(auto module, ParseModule(kProgram, "fake_path.x",
+                                                    "the_module", file_table));
+  XLS_ASSERT_OK_AND_ASSIGN(ConstantDef * top_member,
+                           module->GetMemberOrError<ConstantDef>("MOL2"));
+  XLS_ASSERT_OK_AND_ASSIGN(AstNode * clone, CloneAst(top_member));
+  EXPECT_EQ("const MOL2 = MOL[:];", clone->ToString());
+  XLS_ASSERT_OK(VerifyClone(top_member, clone, file_table));
+}
+
 TEST(AstClonerTest, PreserveTypeDefinitionsReplacer) {
   constexpr std::string_view kProgram =
       R"(

xls/dslx/type_system/typecheck_module_test.cc

@@ -468,6 +468,20 @@ proc Bar {
   XLS_EXPECT_OK(Typecheck(kProgram));
 }
 
+// Note: this previously caused a fatal error as the slice in this position is
+// handled in the AstCloner.
+TEST(TypecheckErrorTest, ProcWithSliceOfNumber) {
+  constexpr std::string_view kProgram = R"(
+proc o {
+  h: chan<u2[0[:]]> in;
+  config(h: chan<u3> out) {}
+}
+)";
+  EXPECT_THAT(Typecheck(kProgram),
+              StatusIs(absl::StatusCode::kInvalidArgument,
+                       HasSubstr("Procs must define `init`")));
+}
+
 TEST(TypecheckTest, ProcWithImplAsProcMemberInTuple) {
   constexpr std::string_view kProgram = R"(
 proc Foo {