Type narrowing for ellipsis via 'is ...' failing in 1.9.0

[dhirving]

Bug Report

As of mypy 1.9.0, narrowing of unions containing EllipsisType via the expressions if x is ... or if x is Ellipsis no longer works. In both branches of the if statement, the type of x still contains EllipsisType instead of being narrowed.

This narrowed as expected in 1.8.0.

As a workaround, it appears that if instanceof(x, EllipsisType) still works as expected.

To Reproduce

Run mypy against the following snippet:

from types import EllipsisType

x: int | EllipsisType

if x is ...:
    pass
else:
    y = x + 1

A more complicated reproduction of this issue can be seen in our repo here: https://github.com/lsst/daf_butler/actions/runs/8209898890/job/22456322445?pr=973

Expected Behavior

mypy 1.8.0 shows no errors because x was narrowed to int in the else clause:

Success: no issues found in 1 source file

Actual Behavior
mypy 1.9.0 reports an error instead:

tmp.py:8: error: Unsupported operand types for + ("EllipsisType" and "int")  [operator]
tmp.py:8: note: Left operand is of type "int | EllipsisType"
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.9.0
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.11.7

[JelleZijlstra]

Probably related to python/typeshed#11223. I suspect some of these need to be updated:

% git grep builtins.ellipsis
mypy/checkexpr.py:        return self.named_type("builtins.ellipsis")
mypy/stubtest.py:                and proper_type.type.fullname == "builtins.ellipsis"
mypy/types.py:            or self.type.fullname == "builtins.ellipsis"
mypy/typeshed/stdlib/marshal.pyi:    | builtins.ellipsis
test-data/unit/check-unions.test:        reveal_type(x) # N: Revealed type is "builtins.ellipsis"
test-data/unit/typexport-basic.test:EllipsisExpr(2) : builtins.ellipsis

[hauntsaninja]

@dhirving Thank you for the issue. I'll add https://github.com/lsst/daf_butler to mypy_primer so we detect regressions against it

[dhirving]

Thanks for the prompt fix! I replied to the ticket on mypy_primer as well -- we'd love to be included in the regression suite.