Ad hoc unions: type of the "single backing field" can be configured

Author: PawelGerr

.claude/CLAUDE.md

@@ -98,7 +98,7 @@ All types must be declared as `partial`. Source generators produce: factory meth
 - Factory methods: when any member triggers factory method generation (type parameter, interface, `System.Object`, or duplicate type), factory methods (`Create{MemberName}`) are generated for **all** members — not just the triggering ones. Constructors retain their configured access modifier; conversion operators are still generated for eligible members. Use `FactoryMethodGeneration` to override: `None` suppresses all factory methods (even for type parameters/duplicates), `Always` generates for all members unconditionally.
 - `allows ref struct` is **not supported** on ad-hoc union type parameters (TTRESG073). Ref structs cannot be boxed, which conflicts with equality, `Value` property, and Switch/Map delegate patterns.
 - Stateless types (`TXIsStateless = true`): store only discriminator, not instance. Prefer structs.
-- Backing fields: with 2+ distinct non-stateless reference types, reference types auto-share a single `object? _obj` field (value types keep typed fields). `UseSingleBackingField = true` forces all types (including value types, with boxing) into `_obj`.
+- Backing fields: with 2+ distinct non-stateless reference types, reference types auto-share a single `object? _obj` field (value types keep typed fields). `UseSingleBackingField = true` forces all types (including value types, with boxing) into `_obj`. `SingleBackingFieldType = typeof(TBase)` additionally types `_obj` and `Value` as `TBase` (implies `UseSingleBackingField = true`); stateless struct members use a `private static readonly` cached boxed default to avoid per-instance allocation. Supports `TypeParamRef1`–`TypeParamRef5` placeholders for generic unions.
 
 **Regular Unions** (`[Union]`):
 
@@ -116,6 +116,7 @@ Several attribute settings cascade to other settings. The AI assistant must acco
 - **`EmptyStringInFactoryMethodsYieldsNull = true`**: Forces `NullInFactoryMethodsYieldsNull = true`
 - **`TXIsStateless = true`** (Unions): Automatically sets `TXIsNullableReferenceType = true` for reference types
 - **`UseSingleBackingField = true`** (Ad-hoc Unions): Forces all member types into a single `object? _obj` backing field. Without this setting, the generator already auto-merges reference types into `_obj` when there are 2+ distinct non-stateless reference types; this setting additionally forces value types into `_obj` (causing boxing)
+- **`SingleBackingFieldType` set** (Ad-hoc Unions): Forces `UseSingleBackingField = true`. `typeof(object)` is normalized to "not set" but still triggers the cascade. Backing field and `Value` are typed as the specified base type instead of `object`. Stateless struct members are stored via a `private static readonly` cached boxed default; stateless reference types leave `_obj` null. For struct unions, `Value` throws `InvalidOperationException` on `default(TUnion)` (discriminator check on every access), matching `AsTx`/`Switch`/`Map`. Conflict with explicit `UseSingleBackingField = false` produces TTRESG075
 - **`ConstructorAccessModifier`** (Unions): Also controls accessibility of implicit conversion operators and factory method accessibility
 - **`FactoryMethodGeneration`** (Ad-hoc Unions): `None` suppresses all factory methods — including for type parameters and duplicates (user must provide custom creation methods). `Always` generates factory methods for all members even without triggers. `Default` auto-detects based on trigger conditions (type parameter, interface, `System.Object`, duplicate type) and generates for all members when any trigger is present
 

.serena/project.yml

@@ -1,57 +1,20 @@
-# whether to use the project's gitignore file to ignore files
-# Added on 2025-04-07
+
+# whether to use project's .gitignore files to ignore files
 ignore_all_files_in_gitignore: true
-# list of additional paths to ignore
-# same syntax as gitignore, so you can use * and **
-# Was previously called `ignored_dirs`, please update your config if you are using that.
-# Added (renamed)on 2025-04-07
+
+# list of additional paths to ignore in this project.
+# Same syntax as gitignore, so you can use * and **.
+# Note: global ignored_paths from serena_config.yml are also applied additively.
 ignored_paths: []
 
 # whether the project is in read-only mode
 # If set to true, all editing tools will be disabled and attempts to use them will result in an error
 # Added on 2025-04-18
 read_only: false
 
-
-# list of tool names to exclude. We recommend not excluding any tools, see the readme for more details.
-# Below is the complete list of tools for convenience.
-# To make sure you have the latest list of tools, and to view their descriptions, 
-# execute `uv run scripts/print_tool_overview.py`.
-#
-#  * `activate_project`: Activates a project by name.
-#  * `check_onboarding_performed`: Checks whether project onboarding was already performed.
-#  * `create_text_file`: Creates/overwrites a file in the project directory.
-#  * `delete_lines`: Deletes a range of lines within a file.
-#  * `delete_memory`: Deletes a memory from Serena's project-specific memory store.
-#  * `execute_shell_command`: Executes a shell command.
-#  * `find_referencing_code_snippets`: Finds code snippets in which the symbol at the given location is referenced.
-#  * `find_referencing_symbols`: Finds symbols that reference the symbol at the given location (optionally filtered by type).
-#  * `find_symbol`: Performs a global (or local) search for symbols with/containing a given name/substring (optionally filtered by type).
-#  * `get_current_config`: Prints the current configuration of the agent, including the active and available projects, tools, contexts, and modes.
-#  * `get_symbols_overview`: Gets an overview of the top-level symbols defined in a given file.
-#  * `initial_instructions`: Gets the initial instructions for the current project.
-#     Should only be used in settings where the system prompt cannot be set,
-#     e.g. in clients you have no control over, like Claude Desktop.
-#  * `insert_after_symbol`: Inserts content after the end of the definition of a given symbol.
-#  * `insert_at_line`: Inserts content at a given line in a file.
-#  * `insert_before_symbol`: Inserts content before the beginning of the definition of a given symbol.
-#  * `list_dir`: Lists files and directories in the given directory (optionally with recursion).
-#  * `list_memories`: Lists memories in Serena's project-specific memory store.
-#  * `onboarding`: Performs onboarding (identifying the project structure and essential tasks, e.g. for testing or building).
-#  * `prepare_for_new_conversation`: Provides instructions for preparing for a new conversation (in order to continue with the necessary context).
-#  * `read_file`: Reads a file within the project directory.
-#  * `read_memory`: Reads the memory with the given name from Serena's project-specific memory store.
-#  * `remove_project`: Removes a project from the Serena configuration.
-#  * `replace_lines`: Replaces a range of lines within a file with new content.
-#  * `replace_symbol_body`: Replaces the full definition of a symbol.
-#  * `restart_language_server`: Restarts the language server, may be necessary when edits not through Serena happen.
-#  * `search_for_pattern`: Performs a search for a pattern in the project.
-#  * `summarize_changes`: Provides instructions for summarizing the changes made to the codebase.
-#  * `switch_modes`: Activates modes by providing a list of their names
-#  * `think_about_collected_information`: Thinking tool for pondering the completeness of collected information.
-#  * `think_about_task_adherence`: Thinking tool for determining whether the agent is still on track with the current task.
-#  * `think_about_whether_you_are_done`: Thinking tool for determining whether the task is truly completed.
-#  * `write_memory`: Writes a named memory (for future reference) to Serena's project-specific memory store.
+# list of tool names to exclude.
+# This extends the existing exclusions (e.g. from the global configuration)
+# Find the list of tools here: https://oraios.github.io/serena/01-about/035_tools.html
 excluded_tools: []
 
 # initial prompt for the project. It will always be given to the LLM upon activating the project
@@ -68,18 +31,24 @@ project_name: "Thinktecture.Runtime.Extensions"
 # Set this to a list of mode names to always include the respective modes for this project.
 base_modes:
 
-# list of mode names that are to be activated by default.
-# The full set of modes to be activated is base_modes + default_modes.
-# If the setting is undefined, the default_modes from the global configuration (serena_config.yml) apply.
+# list of mode names that are to be activated by default, overriding the setting in the global configuration.
+# The full set of modes to be activated is base_modes (from global config) + default_modes + added_modes.
+# If the setting is undefined/empty, the default_modes from the global configuration (serena_config.yml) apply.
 # Otherwise, this overrides the setting from the global configuration (serena_config.yml).
+# Therefore, you can set this to [] if you do not want the default modes defined in the global config to apply
+# for this project.
 # This setting can, in turn, be overridden by CLI parameters (--mode).
+# See https://oraios.github.io/serena/02-usage/050_configuration.html#modes
 default_modes:
 
-# list of tools to include that would otherwise be disabled (particularly optional tools that are disabled by default)
+# list of tools to include that would otherwise be disabled (particularly optional tools that are disabled by default).
+# This extends the existing inclusions (e.g. from the global configuration).
+# Find the list of tools here: https://oraios.github.io/serena/01-about/035_tools.html
 included_optional_tools: []
 
 # fixed set of tools to use as the base tool set (if non-empty), replacing Serena's default set of tools.
 # This cannot be combined with non-empty excluded_tools or included_optional_tools.
+# Find the list of tools here: https://oraios.github.io/serena/01-about/035_tools.html
 fixed_tools: []
 
 # the encoding used by text files in the project
@@ -88,21 +57,26 @@ encoding: utf-8
 
 
 # list of languages for which language servers are started; choose from:
-#   al                  bash                clojure             cpp                 csharp
-#   csharp_omnisharp    dart                elixir              elm                 erlang
-#   fortran             fsharp              go                  groovy              haskell
-#   java                julia               kotlin              lua                 markdown
-#   matlab              nix                 pascal              perl                php
-#   powershell          python              python_jedi         r                   rego
-#   ruby                ruby_solargraph     rust                scala               swift
-#   terraform           toml                typescript          typescript_vts      vue
-#   yaml                zig
+#   al                  angular             ansible             bash                clojure
+#   cpp                 cpp_ccls            crystal             csharp              csharp_omnisharp
+#   dart                elixir              elm                 erlang              fortran
+#   fsharp              go                  groovy              haskell             haxe
+#   hlsl                html                java                json                julia
+#   kotlin              lean4               lua                 luau                markdown
+#   matlab              msl                 nix                 ocaml               pascal
+#   perl                php                 php_phpactor        powershell          python
+#   python_jedi         python_ty           r                   rego                ruby
+#   ruby_solargraph     rust                scala               scss                solidity
+#   swift               systemverilog       terraform           toml                typescript
+#   typescript_vts      vue                 yaml                zig
 #   (This list may be outdated. For the current list, see values of Language enum here:
 #   https://github.com/oraios/serena/blob/main/src/solidlsp/ls_config.py
 #   For some languages, there are alternative language servers, e.g. csharp_omnisharp, ruby_solargraph.)
 # Note:
 #   - For C, use cpp
 #   - For JavaScript, use typescript
+#   - For Angular projects, use angular (subsumes typescript+html; requires `npm install` in the project root)
+#   - For SCSS / Sass / plain CSS, use scss (some-sass-language-server handles all three)
 #   - For Free Pascal/Lazarus, use pascal
 # Special requirements:
 #   Some languages require additional setup/installations.
@@ -148,3 +122,51 @@ ignored_memory_patterns: []
 # Have a look at the docstring of the constructors of the LS implementations within solidlsp (e.g., for C# or PHP) to see which options are available.
 # No documentation on options means no options are available.
 ls_specific_settings: {}
+
+# list of mode names to be activated additionally for this project, e.g. ["query-projects"]
+# The full set of modes to be activated is base_modes (from global config) + default_modes + added_modes.
+# See https://oraios.github.io/serena/02-usage/050_configuration.html#modes
+added_modes:
+
+# list of additional workspace folder paths for cross-package reference support (e.g. in monorepos).
+# Paths can be absolute or relative to the project root.
+# Each folder is registered as an LSP workspace folder, enabling language servers to discover
+# symbols and references across package boundaries.
+# Currently supported for: TypeScript.
+# Example:
+#   additional_workspace_folders:
+#     - ../sibling-package
+#     - ../shared-lib
+additional_workspace_folders: []
+from the global configuration.
+symbol_info_budget:
+
+# The language backend to use for this project.
+# If not set, the global setting from serena_config.yml is used.
+# Valid values: LSP, JetBrains
+# Note: the backend is fixed at startup. If a project with a different backend
+# is activated post-init, an error will be returned.
+language_backend:
+
+# list of regex patterns which, when matched, mark a memory entry as read‑only.
+# Extends the list from the global configuration, merging the two lists.
+read_only_memory_patterns: []
+
+# line ending convention to use when writing source files.
+# Possible values: unset (use global setting), "lf", "crlf", or "native" (platform default)
+# This does not affect Serena's own files (e.g. memories and configuration files), which always use native line endings.
+line_ending:
+
+# list of regex patterns for memories to completely ignore.
+# Matching memories will not appear in list_memories or activate_project output
+# and cannot be accessed via read_memory or write_memory.
+# To access ignored memory files, use the read_file tool on the raw file path.
+# Extends the list from the global configuration, merging the two lists.
+# Example: ["_archive/.*", "_episodes/.*"]
+ignored_memory_patterns: []
+
+# advanced configuration option allowing to configure language server-specific options.
+# Maps the language key to the options.
+# Have a look at the docstring of the constructors of the LS implementations within solidlsp (e.g., for C# or PHP) to see which options are available.
+# No documentation on options means no options are available.
+ls_specific_settings: {}

Directory.Build.props

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <Copyright>(c) $([System.DateTime]::Now.Year), Pawel Gerr. All rights reserved.</Copyright>
-    <VersionPrefix>10.2.0</VersionPrefix>
+    <VersionPrefix>10.3.0</VersionPrefix>
     <Authors>Pawel Gerr</Authors>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <PackageProjectUrl>https://github.com/PawelGerr/Thinktecture.Runtime.Extensions</PackageProjectUrl>

docs

@@ -64,6 +64,7 @@ public sealed class ThinktectureRuntimeExtensionsAnalyzer : DiagnosticAnalyzer
       DiagnosticsDescriptors.ComparisonAndEqualityOperatorsMismatch,
       DiagnosticsDescriptors.UseSwitchMapWithStaticLambda,
       DiagnosticsDescriptors.TypeParamRefRequiresNotnullConstraint,
+      DiagnosticsDescriptors.SingleBackingFieldTypeConflictsWithUseSingleBackingField,
    ];
 
    /// <inheritdoc />
@@ -165,7 +166,7 @@ private void AnalyzeNamedTypes(SymbolAnalysisContext context)
 
          if (adHocUnionAttribute is not null)
          {
-            ValidateAdHocUnion(context, type);
+            ValidateAdHocUnion(context, type, adHocUnionAttribute);
             ValidateObjectFactories(context, type, objectFactoryAttributes, false);
 
             needsObjectFactoryHandling = false;
@@ -544,7 +545,8 @@ private static void AnalyzeSwitchMapLambdas(
 
    private static void ValidateAdHocUnion(
       SymbolAnalysisContext context,
-      INamedTypeSymbol type)
+      INamedTypeSymbol type,
+      AttributeData adHocUnionAttribute)
    {
       if (type.IsRecord || type.TypeKind is not (TypeKind.Class or TypeKind.Struct))
       {
@@ -558,6 +560,22 @@ private static void ValidateAdHocUnion(
 
       CheckConstructors(context, type, mustBePrivate: false, canHavePrimaryConstructor: false);
       TypeMustBePartial(context, type);
+
+      // TTRESG075: explicit 'UseSingleBackingField = false' conflicts with 'SingleBackingFieldType'.
+      var hasSingleBackingFieldType = adHocUnionAttribute.FindSingleBackingFieldType() is not null;
+      var useSingleBackingField = adHocUnionAttribute.FindUseSingleBackingField();
+
+      if (hasSingleBackingFieldType && useSingleBackingField == false)
+      {
+         var location = adHocUnionAttribute.ApplicationSyntaxReference?.GetSyntax(context.CancellationToken).GetLocation()
+                        ?? type.GetTypeIdentifierLocation(context.CancellationToken);
+
+         ReportDiagnostic(
+            context,
+            DiagnosticsDescriptors.SingleBackingFieldTypeConflictsWithUseSingleBackingField,
+            location,
+            BuildTypeName(type));
+      }
    }
 
    private static void ValidateRegularUnion(