Fix serialization of unions with nested PEP 695 type aliases

[PraneelBhatia]

Fixes #330

When a union contains a PEP 695 type alias as one of its members, serializer generation crashed with TypeError: issubclass() arg 1 must be a class, because pack_union passed the raw TypeAliasType object to issubclass(get_type_origin(type_arg), Collection). The deserialization path had the symmetric problem: UnionUnpackerBuilder generated type-match conditions from the alias's __name__, which raised NameError at decode time.

Changes:

• Added a resolve_type_alias_type() helper to mashumaro/core/meta/helpers.py that fully resolves TypeAliasType objects, including parameterized aliases (type ListOf[T] = list[T]) via substitute_type_params.
• Applied it to union members in pack_union and UnionUnpackerBuilder._add_body.
• Refactored the existing alias-unwrapping branches in pack_special_typing_primitive and unpack_special_typing_primitive to use the same helper (behavior unchanged).
• Added regression tests covering the issue's repro, the TO_DICT_ADD_OMIT_NONE_FLAG variant with eager and lazy compilation, codecs, and parameterized aliases as union members. All fail before the fix.

[Copilot]

Pull request overview

Note

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This PR improves Mashumaro’s support for PEP 695 TypeAliasType when aliases are used inside Union/| types (including parameterized aliases), and adds regression tests covering dict serialization, omit-none behavior, and codecs.

Changes:

• Add resolve_type_alias_type() helper to centralize TypeAliasType unwrapping (including parameter substitution for parameterized aliases).
• Apply alias resolution in pack/unpack union handling and in special typing primitive paths.
• Add tests reproducing issue #330 and additional union+alias scenarios.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
tests/test_pep_695.py	Adds regression tests for TypeAliasType nested in unions (incl. omit-none flag + codecs).
mashumaro/core/meta/types/unpack.py	Resolves type aliases before selecting union unpackers and consolidates alias handling.
mashumaro/core/meta/types/pack.py	Resolves type aliases before selecting union packers and consolidates alias handling.
mashumaro/core/meta/helpers.py	Introduces resolve_type_alias_type() helper for reuse across pack/unpack.

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[PraneelBhatia]

Added cycle protection in 829f7c8: the resolver tracks seen values and bounds the number of resolution steps, raising TypeError: Cannot resolve recursive type alias ... for both exact cycles (type A = B; type B = A) and non-repeating chains (type G[T] = G[list[T]]). The seen set tracks values rather than id()s because each substitution builds a new (equal) object for parameterized cycles; is_hashable() guards unhashable values and the step bound is the backstop. Raising a targeted error seemed more useful than returning the unresolved alias, which would just push the non-termination into the packer/unpacker dispatch. Tests cover both shapes, including at dataclass creation time.

[PraneelBhatia]

Replaced the silent truncation in 829f7c8: missing trailing parameters now fall back to their PEP 696 defaults when defined (TypeAliasType subscription doesn't auto-fill defaults, so Pair[int] with TypeVar("V", default=str) previously leaked an unsubstituted ~V into the resolved type), and a genuine mismatch raises TypeError: Too few/Too many arguments for ...; actual N, expected M in the same format as the existing generics arity check. The check is skipped for TypeVarTuple/ParamSpec parameters since their subscription arity is flexible. One behavior note: invalid annotations like Pair[int] without a default used to serialize with a half-substituted value and now fail at class creation — happy to relax that if you'd rather keep them lenient.

[codecov]

Codecov Report

❌ Patch coverage is 99.29577% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.91%. Comparing base (e6894ef) to head (829f7c8).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
mashumaro/core/meta/helpers.py	96.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #331      +/-   ##
==========================================
- Coverage   99.92%   99.91%   -0.01%     
==========================================
  Files         101      101              
  Lines       12007    12131     +124     
==========================================
+ Hits        11998    12121     +123     
- Misses          9       10       +1     

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