Fix NotImplementedError on heterogeneous union model fields

[wbarnha]

Description

Defining a faust.Record with a heterogeneous union field — one whose members are neither all Model subclasses nor all scalar "literal" types (str/bytes/float/int) — raised NotImplementedError: Union of types ... not supported at class-definition time. Minimal reproduction (from faust-streaming/mode#80):

from faust import Record

class SensitiveInfo(Record):
    data: str | list | dict | None
    meta: dict

Root cause

faust's type-expression compiler (faust/models/typing.py) dispatches unions to UnionNode. UnionNode._maybe_unroll_union collapses two union shapes to a single node — all-Model unions, and all-LITERAL_TYPES unions (which is why Union[str, int, float] already compiles to a bare pass-through). Every other union fell through to UnionNode.build, which unconditionally raised.

That path became reachable for X | Y annotations once mode-streaming began recognizing PEP 604 unions (mode 0.5.3): str | list | dict | None now registers as a union and reaches this node, whereas an unrecognized X | Y previously fell through to a literal pass-through. The exception is raised by faust's compiler, not by mode — this is the faust-side fix for the boundary characterized in faust-streaming/mode#81.

Fix

A heterogeneous union has no unambiguous coercion target, so UnionNode.build now emits a pass-through expression (the already-deserialized value is returned unchanged), mirroring the existing all-literal handling. Optional/None members keep their optional wrapping ((a if a is not None else None)). This restores the pre-0.5.3 behavior and is strictly more permissive than the previous hard failure — no existing test asserted the raise.

Tests

• TypeExpression-level cases added to the parametrized CASES: Union[str, list, dict] → a, and Union[str, list, dict, None] → (a if a is not None else None).
• End-to-end Record regression tests for both the typing.Union and PEP 604 (|) spellings, asserting values (including None) round-trip unchanged.

All 108 tests/{functional,unit}/models tests pass locally; isort/black/flake8 clean.

Fixes the crash reported in faust-streaming/mode#80.

🤖 Generated with Claude Code

https://claude.ai/code/session_01BnTZQEXqpedyrFAgwRnbsA

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.06%. Comparing base (706b502) to head (fb21cf8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #747      +/-   ##
==========================================
+ Coverage   95.98%   96.06%   +0.07%     
==========================================
  Files         103      103              
  Lines       11069    11069              
  Branches     1191     1191              
==========================================
+ Hits        10625    10633       +8     
+ Misses        350      345       -5     
+ Partials       94       91       -3     

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