fix: handle union-bound TypeVar in type[T] callable analysis

[Bahtya]

Problem

When a TypeVar T has a union bound (e.g. T: bool | int | float | str), calling a value of type type[T] produces a false positive:

T = TypeVar("T", bound="bool | int | float | str")

def func(ftype: type[T], value: str | None) -> T:
    if value is None:
        return ftype()      # error: Incompatible return value type
    return ftype(value)     # error: Incompatible return value type

The error is: Incompatible return value type (got "int | float | str", expected "T").

With a single-type bound (e.g. T: int) this works correctly.

Root Cause

Two issues in checkexpr.py and types.py:

1. analyze_type_type_callee (checkexpr.py): When handling TypeVarType, it recurses on the upper bound. For a union bound, this returns a UnionType of callables, but the code only replaced return types with the TypeVar for CallableType and Overloaded — not UnionType. The union members kept their original return types (e.g. int, float) instead of T.

2. CallableType.type_object() (types.py): When a TypeVar's upper bound is a union, get_proper_type(ret.upper_bound) returns a UnionType, which hits the final assert isinstance(ret, Instance) and crashes.

Solution

1. Added a _replace_callable_return_type helper and a UnionType branch in the TypeVarType handling in analyze_type_type_callee, so each callable in the union gets its return type replaced with the TypeVar.

2. Added a UnionType case in CallableType.type_object() that resolves to the first Instance in the union (this is only used for is_protocol checks, which don't apply to union-bound TypeVars).

Testing

Verified with the exact reproduction case from the issue — now passes with no errors. Also tested:

• Single-type bounds still work (no regression)
• Unbounded TypeVars still work
• Correct type inference: reveal_type(func(bool, 'True')) shows bool, not the full union
• Self-check passes for both modified files (mypy_self_check.ini)

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[Bahtya]

Hi mypy maintainers! Friendly ping on this PR.

To summarize for ease of review:

• All CI checks pass (including mypy_primer, mypyc, self-check across Python 3.10–3.14)
• mypy_primer result: No effect on the open source corpus — clean ✅
• The fix is targeted: two small changes in checkexpr.py and types.py to handle the UnionType case for union-bound TypeVars in type[T] callable analysis.
• Fixes a false positive where calling ftype() or ftype(value) on a union-bound TypeVar parameter incorrectly produced Incompatible return value type errors.

Happy to make any adjustments if needed. Thanks for your time!