Revisit the body of a loop if the number of partial types has changed.

[tyralla]

Fixes #5423

[hauntsaninja]

I went through the primer and this is looking great. 3-5 cases where ideally mypy would do something different, but may be technically unrelated to this PR (didn't look).

operator: true positive
*prefect: false positive, seems like we should narrow from tuple to str on line 220
manticore: true positive
*pylint: false positive, can't tell why mypy wants an explicit annotation
xarray: true positive from typing perspective (typeguard guarantee isn't encoded in signature)
sympy: true positive, should assert
comtypes: true positive, heterogeneous list that is mutated so can't be a tuple[int, x, y, z]
aiortc: true positive, not sure where _data comes from, is maybe patched on
*httpx-caching: false positive, should infer int | None or require explicit annotation
jax: true positive, true positive
flake8: true positive, should assert
*poetry: probably should infer Term | None or require an explicit annotation
dulwich: true positive, should assert
*pyodide: false positive, not sure what's going on there

[tyralla]

@hauntsaninja: Thank you very much for looking so thoroughly through the primer!

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

operator (https://github.com/canonical/operator)
+ ops/lib/__init__.py:219: error: Argument 1 to "_Missing" has incompatible type "dict[str | Any, object]"; expected "bool"  [arg-type]

manticore (https://github.com/trailofbits/manticore)
+ tests/wasm/json2mc.py:103: error: Invalid index type "int | None" for "list[Module]"; expected type "SupportsIndex"  [index]

prefect (https://github.com/PrefectHQ/prefect)
+ src/prefect/settings/legacy.py:105: note:     Expected:
+ src/prefect/settings/legacy.py:105: note:         def __hash__() -> int
+ src/prefect/settings/legacy.py:105: note:     Got:
+ src/prefect/settings/legacy.py:105: note:         def __hash__(self: object) -> int
+ src/prefect/cli/cloud/__init__.py:224: error: Incompatible return value type (got "str | tuple[Hashable, str]", expected "str")  [return-value]

pylint (https://github.com/pycqa/pylint)
+ pylint/lint/message_state_handler.py:381: error: Need type annotation for "_ignore_file"  [var-annotated]

xarray (https://github.com/pydata/xarray)
+ xarray/core/indexing.py: note: In member "__init__" of class "VectorizedIndexer":
+ xarray/core/indexing.py:488: error: Item "_arrayfunction[Any, Any]" of "_arrayfunction[Any, Any] | _arrayapi[Any, Any]" has no attribute "ndim"  [union-attr]
+ xarray/core/indexing.py:488: error: Item "_arrayapi[Any, Any]" of "_arrayfunction[Any, Any] | _arrayapi[Any, Any]" has no attribute "ndim"  [union-attr]

sympy (https://github.com/sympy/sympy)
+ sympy/tensor/array/expressions/from_array_to_matrix.py:398: error: Invalid index type "int | None" for "list[Any]"; expected type "SupportsIndex"  [index]
+ sympy/tensor/array/expressions/from_array_to_matrix.py:401: error: Invalid index type "int | None" for "list[Any]"; expected type "SupportsIndex"  [index]

comtypes (https://github.com/enthought/comtypes)
+ comtypes/tools/codegenerator/codegenerator.py:848: error: Unsupported operand types for | ("object" and "int")  [operator]
+ comtypes/tools/codegenerator/codegenerator.py:850: error: Unsupported operand types for | ("object" and "int")  [operator]

aiortc (https://github.com/aiortc/aiortc)
+ src/aiortc/jitterbuffer.py:81: error: "RtpPacket" has no attribute "_data"  [attr-defined]

httpx-caching (https://github.com/johtso/httpx-caching)
+ httpx_caching/_policy.py:491: error: Incompatible types in assignment (expression has type "None", target has type "int")  [assignment]

flake8 (https://github.com/pycqa/flake8)
+ src/flake8/processor.py:217: error: Unsupported operand types for - ("None" and "int")  [operator]
+ src/flake8/processor.py:217: note: Left operand is of type "int | None"

jax (https://github.com/google/jax)
+ jax/_src/interpreters/pxla.py:2249: error: Item "AbstractMesh" of "Mesh | AbstractMesh" has no attribute "abstract_mesh"  [union-attr]
+ jax/_src/interpreters/pxla.py:2253: error: Item "AbstractMesh" of "Mesh | AbstractMesh" has no attribute "abstract_mesh"  [union-attr]
+ jax/_src/checkify.py:262: error: Value of type "int | Array" is not indexable  [index]

poetry (https://github.com/python-poetry/poetry)
+ src/poetry/mixology/partial_solution.py:195: error: Incompatible types in assignment (expression has type "Term | None", variable has type "Assignment | None")  [assignment]

pyodide (https://github.com/pyodide/pyodide)
+ src/py/pyodide/_state.py:21: error: Incompatible types in assignment (expression has type "pyodide._state.<subclass of "ModuleType" and "JsProxy">1", target has type "pyodide._state.<subclass of "ModuleType" and "JsProxy">")  [assignment]

dulwich (https://github.com/dulwich/dulwich)
+ dulwich/config.py:558: error: Unsupported left operand type for + ("None")  [operator]
+ dulwich/config.py:558: note: Left operand is of type "bytes | None"
+ dulwich/config.py:560: error: Unsupported left operand type for + ("None")  [operator]
+ dulwich/config.py:560: note: Left operand is of type "bytes | None"
+ dulwich/config.py:562: error: Unsupported left operand type for + ("None")  [operator]
+ dulwich/config.py:562: note: Left operand is of type "bytes | None"

[tyralla]

I had a look at the false positive for prefect. It seems in fact unrelated to this PR, as it is reproducible without involving loops:

from typing import Union, Tuple

def f() -> bool: ...

u: Union[int, Tuple[int]]
x = None
if f():
    x = u
    if isinstance(x, tuple):
        x = x[0]
reveal_type(x)  # N: Revealed type is "Union[builtins.int, Tuple[builtins.int], None]" 

(Tuple[builtins.int] should be narrowed away.)

[ilevkivskyi]

The fix makes sense to me. I will merge soon unless there are objections. (Please ping me if I forget about this PR)