[DSLX:TS] Cannot index a parameteric-signedness value.

xls/dslx/type_system/deduce.cc

@@ -1314,6 +1314,14 @@ absl::StatusOr<std::unique_ptr<Type>> DeduceIndex(const Index* node,
         ctx->file_table());
   }
 
+  std::optional<BitsLikeProperties> bits_like = GetBitsLike(*lhs_type);
+  if (bits_like.has_value()) {
+    return TypeInferenceErrorStatus(
+        node->span(), lhs_type.get(),
+        "Bits-like value cannot be indexed, value to index is not an array.",
+        ctx->file_table());
+  }
+
   auto* array_type = dynamic_cast<ArrayType*>(lhs_type.get());
   if (array_type == nullptr) {
     return TypeInferenceErrorStatus(node->span(), lhs_type.get(),

xls/dslx/type_system/typecheck_module_test.cc

@@ -1961,18 +1961,28 @@ fn f() -> u32 {
 }
 
 TEST(TypecheckTest, Index) {
+  // Indexing a 4-element bit-constructed array with a constant index.
   XLS_EXPECT_OK(Typecheck("fn f(x: uN[32][4]) -> u32 { x[u32:0] }"));
+
+  // Indexing a 5-element array with a dynamic index.
   XLS_EXPECT_OK(Typecheck("fn f(x: u32[5], i: u8) -> u32 { x[i] }"));
+
+  // Indexing a bit value is not allowed, only arrays.
   EXPECT_THAT(
       Typecheck("fn f(x: u32, i: u8) -> u32 { x[i] }"),
       StatusIs(absl::StatusCode::kInvalidArgument, HasSubstr("not an array")));
 
-  // Use an array as an index.
+  // Indexing a parametric-signedness bit value is not allowed.
+  EXPECT_THAT(
+      Typecheck("fn f(x: xN[false][5], i: u8) -> u1 { x[i] }"),
+      StatusIs(absl::StatusCode::kInvalidArgument, HasSubstr("not an array")));
+
+  // Cannot use an array as an index.
   EXPECT_THAT(Typecheck("fn f(x: u32[5], i: u8[5]) -> u32 { x[i] }"),
               StatusIs(absl::StatusCode::kInvalidArgument,
                        HasSubstr("Index is not bits typed")));
 
-  // Use a signed number as an index.
+  // Cannot use a signed number as an index.
   EXPECT_THAT(Typecheck("fn f(x: u32[5], i: s8) -> u32 { x[i] }"),
               StatusIs(absl::StatusCode::kInvalidArgument,
                        HasSubstr("Index is not unsigned-bits typed")));