When attempting to spawn a proc in a function return an Unimplemented error status instead of an Internal error

xls/dslx/ir_convert/extract_conversion_order.cc

@@ -255,7 +255,10 @@ class InvocationVisitor : public ExprVisitor {
     std::optional<ProcId> proc_id;
     auto maybe_proc = callee_info->callee->proc();
     if (maybe_proc.has_value()) {
-      XLS_RET_CHECK(proc_id_.has_value()) << "Functions cannot spawn procs.";
+      if (!proc_id_.has_value()) {
+        return absl::UnimplementedError(
+            absl::StrFormat("Functions cannot spawn procs."));
+      }
 
       // Only count `next` as a new instance, so that `config` and `next` have
       // the same ID. This assumes that we call a proc's `config` and `next` in

xls/dslx/ir_convert/function_converter.cc

@@ -3333,13 +3333,16 @@ absl::Status FunctionConverter::HandleSpawn(const Spawn* node) {
   }
   ProcBuilder* builder_ptr =
       dynamic_cast<ProcBuilder*>(function_builder_.get());
-  XLS_RET_CHECK_NE(builder_ptr, nullptr)
-      << "Spawn nodes should only be encountered during proc conversion; "
-         "we seem to be in function conversion.";
+  if (builder_ptr == nullptr) {
+    return absl::UnimplementedError(
+        absl::StrFormat("IrConversionError: %s Functions cannot spawn procs.",
+                        node->span().ToString(file_table())));
+  }
   if (current_fn_tag_ != FunctionTag::kProcConfig) {
-    return IrConversionErrorStatus(
-        node->span(), "Procs can only be spawned in a proc `config` method.",
-        file_table());
+    return absl::UnimplementedError(absl::StrFormat(
+        "IrConversionError: %s Procs can only be spawned in a proc `config` "
+        "method.",
+        node->span().ToString(file_table())));
   }
 
   const Invocation* invocation = node->config();

xls/dslx/ir_convert/ir_converter_test.cc

@@ -3420,8 +3420,52 @@ fn main() {
   auto import_data = CreateImportDataForTest();
   EXPECT_THAT(
       ConvertOneFunctionForTest(program, "main", import_data, kNoPosOptions),
-      StatusIs(absl::StatusCode::kInternal,
-               HasSubstr("Functions cannot spawn procs")));
+      StatusIs(absl::StatusCode::kUnimplemented,
+               HasSubstr("Functions cannot spawn procs.")));
+}
+
+TEST_P(IrConverterWithBothTypecheckVersionsTest,
+       InvalidSpawnProcScopedChannel) {
+  constexpr std::string_view program = R"(
+proc p {
+  init { () }
+  config() { () }
+  next(state: ()) { state }
+}
+
+fn main() {
+  spawn p();
+}
+)";
+  auto import_data = CreateImportDataForTest();
+  EXPECT_THAT(ConvertOneFunctionForTest(program, "main", import_data,
+                                        kProcScopedChannelOptions),
+              StatusIs(absl::StatusCode::kUnimplemented,
+                       HasSubstr("Functions cannot spawn procs.")));
+}
+
+TEST_P(IrConverterWithBothTypecheckVersionsTest, InvalidSpawnInNextProcScoped) {
+  constexpr std::string_view program = R"(
+proc p {
+  init { () }
+  config() { () }
+  next(state: ()) { state }
+}
+
+proc main {
+  init { () }
+  config() { () }
+  next(state: ()) {
+    spawn p();
+  }
+}
+)";
+  auto import_data = CreateImportDataForTest();
+  EXPECT_THAT(ConvertOneFunctionForTest(program, "main", import_data,
+                                        kProcScopedChannelOptions),
+              StatusIs(absl::StatusCode::kUnimplemented,
+                       HasSubstr("Procs can only be spawned in a proc `config` "
+                                 "method.")));
 }
 
 TEST_P(IrConverterWithBothTypecheckVersionsTest,