[Divyansh151005]

Summary

Fixes #5147

isinstance with a type[T] classinfo narrowed a union-typed value to Never:

def fn[T](value: int | str, cls: type[T]):
    if isinstance(value, cls):
        reveal_type(value)  # before: Never, after: (int & T) | (str & T)

Narrowing to Never makes the rest of the branch look unreachable, so errors inside it are silently suppressed.

Root cause. When the narrowed value is a union, narrow_isinstance_from_definite intersects each member with the target using IntersectFallback::Never. That is intentional for concrete targets, so isinstance(x, Iterable) on int | list[int] narrows to just list[int]. A type variable target, however, is neither a subtype nor a supertype of any concrete member, so every member hit the fallback and the whole union became Never. (The non-union case already used the Right fallback and produced int & T.)

Fix. Use the regular Right fallback when the target is a type variable. Each member then intersects with T, producing (int & T) | (str & T). This is consistent with how we already narrow x: T against concrete classes (e.g. (float & T) | (int & T) in test_typevar_isinstance_narrow_else) and with what ty reports. Members that cannot overlap with the TypeVar's bound are still removed, since intersect_impl compares disjoint_bases and disjoint_base uses the bound: with T: int, int | str narrows to T.

The Never fallback for concrete targets is unchanged.

• I am a human, and I fully understand the code I am submitting.

Test Plan

Added test_isinstance_union_with_typevar_classinfo to pyrefly/lib/test/narrow.rs, covering:

• the issue's example: int | str narrowed by type[T] → (int & T) | (str & T); the negative branch stays int | str
• a bounded TypeVar (T: int): int | str → T (the str member is dropped via the bound)
• the non-union case: int → int & T (unchanged)
• returning the narrowed value from a -> T function type-checks

Before the fix:

$ cargo test -p pyrefly --lib test_isinstance_union_with_typevar_classinfo
 INFO revealed type: Never [reveal-type]
4724 |         reveal_type(value)  # E: revealed type: (int & T) | (str & T)
test test::narrow::test_isinstance_union_with_typevar_classinfo ... FAILED

After:

$ cargo test -p pyrefly --lib test_isinstance_union_with_typevar_classinfo
test test::narrow::test_isinstance_union_with_typevar_classinfo ... ok

$ cargo test -p pyrefly --lib -- test::narrow:: test::pattern_match:: test::generic_restrictions:: test::type_guard test::typeis test::generic_basic:: test::simple:: test::type_var
test result: ok. 850 passed; 0 failed

$ cargo test -p pyrefly --lib -- --skip commands::coverage --skip lsp_interaction
test result: ok. 8762 passed; 0 failed

$ python3 test.py --no-test --no-tensor-shapes --no-conformance --no-jsonschema
Running formatting... Running linting... Running extension tests... OK

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D125878285. (Because this pull request was imported automatically, there will not be any future comments.)

[github-actions]

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