Feature: Implement variadic Union[*Ts] for unpacked comprehensions

[AliiiBenn]

Description

Implement variadic Union[*Ts] as specified in PEP 827.

From the PEP:

Union[*Ts]: Union will become able to take variadic arguments, so that it can take unpacked comprehension arguments.

This enables using Union with unpacked type comprehensions like:

type MyUnion = Union[*[int for t in Iter[SomeTuple]]]

Implementation Notes

Current State

• Python's typing.Union already accepts multiple arguments
• The PEP proposes allowing *Ts unpacking syntax

Approach

This is more of a syntax/semantic change than a new evaluator. The implementation might involve:

1. Type checker changes: The type checker would need to understand Union[*Ts] syntax
2. Runtime support: At runtime, this would need to:
   • Detect when Union is subscripted with unpacked args
   • Flatten the unpacked tuple into individual Union arguments

Challenges

• Python's __class_getitem__ doesn't naturally support *args unpacking
• Would need to intercept the unpacking at the type alias level
• May require changes to how typing.Union handles its arguments

Potential Implementation

# In typing.py
class Union[*Ts]:
    pass

# In _eval_operators.py
# The evaluator would need to handle unpacking the Ts tuple
@type_eval.register_evaluator(Union)
def _eval_Union(*types, ctx):
    # Unpack and evaluate each type
    evaluated = [_eval_types(t, ctx) for t in types]
    return typing.Union[tuple(evaluated)]

However, this requires proper variadic generic support in Python 3.14+.

Test Cases

• Basic variadic Union: Union[int, str, float]
• With unpacked comprehension: Union[*[int for t in Iter[tuple[int, str, float]]]]
• With nested types

Status

• Research Python 3.14 variadic generic support
• Determine if this is a type checker or runtime feature
• Prototype implementation
• Add tests
• Update documentation

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Related PEP 827 Section: Union processing (lines 628-629)