Do not consider bare TypeVar not overlapping with None for reachability analysis

[sterliakov]

Fixes #18126.

Simply allowing such intersection was insufficient: existing binder logic widened the type to T | None after the is None check.

This PR extends the binder logic to prevent constructing a union type when all conditional branches are reachable and contain no assignments: checking if isinstance(something, Something) does not change the type of something after the end of the if block.

[sterliakov]

CI failed with the Windows problem I faced multiple times before, the rest is green.

Primer diff is amazing this time! Every diff line seems to be an improvement.

• jinja2: unrelated meet-vs-join problem. Minimal repro (playground):

  class A: pass
  class B: pass
  
  foos: tuple[A, ...] | tuple[B, ...]
  
  for _, foo in enumerate(foos):
      reveal_type(foo)  # N: Revealed type is "builtins.object"

• prefect: the error is correct. I don't know why it wasn't emitted previously, though: there's assignment of different types to an unannotated variable in if/else branches.
• sphinx/search/__init__.py: bug in docutils-stubs (children is declared in Node, but not in stubs), and mypy is correct now.
• sphinx/ext/graphviz.py: correct! This attribute isn't declared on any of the classes. Still don't know how it relates to this change, though...
• schema_salad: correct! self.name is Any | None and is passed to a function expecting str.
• kopf: another unrelated bug:( Minimal repro (playground with sync/async comparison):

  from typing import Any, Callable, TypeVar
  
  T = TypeVar("T")
  
  async def call(fn: Callable[[], T]) -> T:
      return fn()
  
  make_x: Any
  
  async def fn() -> None:
      x: Any | None = None
      x = await call(make_x)
      reveal_type(x)  # N: Revealed type is "Union[Any, None]"

• pytest: correct. There's really no error.
• paroxython: correct, there's a narrowing not supported by mypy
• pandera: fine, undeclared attributes
• cki-lib: correct! It boils down to the following:

  x: Any
  if x is None:
      ...
  do_something_with_not_none(x)

• pygithub: correct! That is really a type error.
• discord: all correct! Especially nice to get rid of Item "object" of the upper bound "Cog | None" - that was strange.
• steam: both correct! Second one suffered from needless type widening after isinstance
• ibis: all correct!

[github-actions]

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

jinja (https://github.com/pallets/jinja)
+ src/jinja2/lexer.py:761: error: "object" not callable  [operator]
+ src/jinja2/lexer.py:781: error: Incompatible types in "yield" (actual type "Tuple[int, object, str]", expected type "Tuple[int, str, str]")  [misc]

prefect (https://github.com/PrefectHQ/prefect)
+ src/prefect/states.py:227: error: Incompatible types in assignment (expression has type "BaseException", variable has type "ResultRecord[Any]")  [assignment]

sphinx (https://github.com/sphinx-doc/sphinx)
+ sphinx/search/__init__.py:628:18: error: "Node" has no attribute "children"  [attr-defined]
+ sphinx/ext/graphviz.py: note: In function "render_dot":
+ sphinx/ext/graphviz.py:307:9: error: Item "StandaloneHTMLBuilder" of "StandaloneHTMLBuilder | LaTeXBuilder | TexinfoBuilder" has no attribute "_graphviz_warned_dot"  [union-attr]
+ sphinx/ext/graphviz.py:307:9: error: Item "LaTeXBuilder" of "StandaloneHTMLBuilder | LaTeXBuilder | TexinfoBuilder" has no attribute "_graphviz_warned_dot"  [union-attr]
+ sphinx/ext/graphviz.py:307:9: error: Item "TexinfoBuilder" of "StandaloneHTMLBuilder | LaTeXBuilder | TexinfoBuilder" has no attribute "_graphviz_warned_dot"  [union-attr]

schema_salad (https://github.com/common-workflow-language/schema_salad)
+ schema_salad/metaschema.py: note: In member "save" of class "EnumSchema":
+ schema_salad/metaschema.py:1882:49: error: Argument 2 to "save_relative_uri" has incompatible type "Any | None"; expected "str"  [arg-type]
+ schema_salad/metaschema.py:1886:49: error: Argument "base_url" to "save" has incompatible type "Any | None"; expected "str"  [arg-type]
+ schema_salad/metaschema.py: note: In member "save" of class "SaladEnumSchema":
+ schema_salad/metaschema.py:5378:51: error: Argument "base_url" to "save" has incompatible type "Any | None"; expected "str"  [arg-type]
+ schema_salad/metaschema.py:5381:49: error: Argument 2 to "save_relative_uri" has incompatible type "Any | None"; expected "str"  [arg-type]
+ schema_salad/metaschema.py:5385:49: error: Argument "base_url" to "save" has incompatible type "Any | None"; expected "str"  [arg-type]
+ schema_salad/metaschema.py:5389:47: error: Argument "base_url" to "save" has incompatible type "Any | None"; expected "str"  [arg-type]
+ schema_salad/metaschema.py:5392:51: error: Argument 2 to "save_relative_uri" has incompatible type "Any | None"; expected "str"  [arg-type]
+ schema_salad/metaschema.py:5395:50: error: Argument 2 to "save_relative_uri" has incompatible type "Any | None"; expected "str"  [arg-type]
+ schema_salad/metaschema.py:5398:50: error: Argument 2 to "save_relative_uri" has incompatible type "Any | None"; expected "str"  [arg-type]
+ schema_salad/metaschema.py:5404:26: error: Argument "base_url" to "save" has incompatible type "Any | None"; expected "str"  [arg-type]
+ schema_salad/metaschema.py:5411:26: error: Argument "base_url" to "save" has incompatible type "Any | None"; expected "str"  [arg-type]
+ schema_salad/metaschema.py:5415:49: error: Argument 2 to "save_relative_uri" has incompatible type "Any | None"; expected "str"  [arg-type]

kopf (https://github.com/nolar/kopf)
+ kopf/_kits/webhooks.py:539: error: Item "None" of "Any | None" has no attribute "public_url"  [union-attr]

pytest (https://github.com/pytest-dev/pytest)
- src/_pytest/_code/code.py:1071: error: Argument 1 to "from_exception" of "ExceptionInfo" has incompatible type "Optional[BaseException]"; expected "BaseException"  [arg-type]
- src/_pytest/_code/code.py:1071: error: Item "None" of "Optional[BaseException]" has no attribute "__traceback__"  [union-attr]
- src/_pytest/_code/code.py:1077: error: Argument 1 to "from_exception" of "ExceptionInfo" has incompatible type "Optional[BaseException]"; expected "BaseException"  [arg-type]
- src/_pytest/_code/code.py:1077: error: Item "None" of "Optional[BaseException]" has no attribute "__traceback__"  [union-attr]

paroxython (https://github.com/laowantong/paroxython)
+ paroxython/flatten_ast.py:401: error: "AST" has no attribute "lineno"  [attr-defined]

pandera (https://github.com/pandera-dev/pandera)
- pandera/api/dataframe/model.py:72: error: Unused "type: ignore" comment  [unused-ignore]
- pandera/api/dataframe/model.py:77: error: Unused "type: ignore" comment  [unused-ignore]

cki-lib (https://gitlab.com/cki-project/cki-lib)
- cki_lib/s3bucket.py:57: error: Item "None" of "Any | None" has no attribute "get_credentials"  [union-attr]

PyGithub (https://github.com/PyGithub/PyGithub)
- github/GithubObject.py:315: error: Unused "type: ignore" comment  [unused-ignore]

discord.py (https://github.com/Rapptz/discord.py)
- discord/ext/commands/core.py:590: error: Unused "type: ignore" comment  [unused-ignore]
- discord/ext/commands/core.py:645: error: Item "object" of the upper bound "Cog | None" of type variable "CogT" has no attribute "cog_command_error"  [union-attr]
- discord/ext/commands/core.py:884: error: Item "object" of the upper bound "Cog | None" of type variable "CogT" has no attribute "cog_before_invoke"  [union-attr]
- discord/ext/commands/core.py:904: error: Item "object" of the upper bound "Cog | None" of type variable "CogT" has no attribute "cog_after_invoke"  [union-attr]
- discord/ext/commands/core.py:1148: error: "type[CogT]" has no attribute "__cog_name__"  [attr-defined]
- discord/ext/commands/core.py:1277: error: Item "object" of the upper bound "Cog | None" of type variable "CogT" has no attribute "cog_check"  [union-attr]

steam.py (https://github.com/Gobot1234/steam.py)
- steam/app.py:130: error: Argument "game_extra_info" to "CMsgClientGamesPlayedGamePlayed" has incompatible type "NameT"; expected "str"  [arg-type]
- steam/ext/commands/commands.py:923: error: Incompatible return value type (got "Command[Any, Any, Any] | MaybeCommandT", expected "MaybeBool | MaybeCommandT")  [return-value]

ibis (https://github.com/ibis-project/ibis)
+ ibis/util.py:137: error: Incompatible return value type (got "tuple[()]", expected "tuple[V]")  [return-value]
- ibis/selectors.py:708: error: Argument 1 to "ColumnIndex" has incompatible type "int | slice[Any, Any, Any] | Iterable[int | str]"; expected "str | int | Slice | tuple[int | str, ...]"  [arg-type]
+ ibis/selectors.py:708: error: Argument 1 to "ColumnIndex" has incompatible type "str | int | slice[Any, Any, Any] | Iterable[int | str]"; expected "str | int | Slice | tuple[int | str, ...]"  [arg-type]
- ibis/expr/types/relations.py:2629: error: Argument 3 to "DropNull" has incompatible type "Sequence[str] | None"; expected "tuple[Value[Any, Columnar], ...] | None"  [arg-type]
+ ibis/expr/types/relations.py:2629: error: Argument 3 to "DropNull" has incompatible type "Sequence[str] | str | None"; expected "tuple[Value[Any, Columnar], ...] | None"  [arg-type]
+ ibis/backends/sqlite/__init__.py:497: error: Item "None" of "Any | Any | Any | Any | Any | None" has no attribute "schema"  [union-attr]
- ibis/backends/duckdb/__init__.py:532: error: Argument 1 to "read_parquet" of "Backend" has incompatible type "str | Any | Path | list[Any] | tuple[Any, ...]"; expected "str | Iterable[str]"  [arg-type]
+ ibis/backends/duckdb/__init__.py:532: error: Argument 1 to "read_parquet" of "Backend" has incompatible type "str | Path | Any"; expected "str | Iterable[str]"  [arg-type]
- ibis/backends/duckdb/__init__.py:536: error: Argument 1 to "read_csv" of "Backend" has incompatible type "str | Any | Path | list[Any] | tuple[Any, ...]"; expected "str | list[str] | tuple[str]"  [arg-type]
+ ibis/backends/duckdb/__init__.py:536: error: Argument 1 to "read_csv" of "Backend" has incompatible type "str | Path | Any"; expected "str | list[str] | tuple[str]"  [arg-type]
- ibis/backends/duckdb/__init__.py:538: error: Argument 1 to "read_postgres" of "Backend" has incompatible type "str | Any | Path | list[Any] | tuple[Any, ...]"; expected "str"  [arg-type]
+ ibis/backends/duckdb/__init__.py:538: error: Argument 1 to "read_postgres" of "Backend" has incompatible type "str | Path | Any"; expected "str"  [arg-type]
- ibis/backends/pyspark/__init__.py:962: error: Argument 1 to "read_parquet" of "Backend" has incompatible type "str | Any | Path | list[Any] | tuple[Any, ...]"; expected "str | Path"  [arg-type]
- ibis/backends/pyspark/__init__.py:966: error: Argument 1 to "read_csv" of "Backend" has incompatible type "str | Any | Path | list[Any] | tuple[Any, ...]"; expected "str | list[str] | tuple[str]"  [arg-type]
+ ibis/backends/pyspark/__init__.py:966: error: Argument 1 to "read_csv" of "Backend" has incompatible type "str | Path | Any"; expected "str | list[str] | tuple[str]"  [arg-type]